SharePoint sites vs. Site Collections - CONFUSED!

Thank you for your answer Dean. I did post it under new thread but it was marked as spam for some reason :(

1. no, a site collection cannot contain another site collection. the site collection is the top level and has functionality that only exists at that level. There is a site (root site) at the top and it can contain sites (sub-sites) that are many levels deep. The functionality at the lower levels is the same for every site (however, this can vary if different site templates are used)

Please create a new thread(s) with your own questions so that they can be more easily addressed by the community.

Thank you all for this discussion. I found a lot of useful information.

Here are couple more questions I didn't find answers to:

1. Are subsites site collections themselves? I can create a sub-sub-site, so I assume they are collections, right?

2. Are there any information on the web what are "modern" and what are "legacy" style site collections? How they are different? I assume if I create a site collection from SP Admin I am creating a "legacy" site collection, and if I create a collection from '+ Create Site' button I am creating "modern" site? Can they be interchanged together, I mean can I create a "modern" sub-site in "legacy" site collection?

We are a large enterprise with 10-12 companies in different business fields (real estate, media, telecommunications, retail, etc). These companies also have another 5 to 50 smaller daughter companies. Since most of the parent companies do not really relate to each other we decided that we need to create separate intranet websites for all of them. Daughter companies will have subsites or no sites all all. We also would like to have one major intranet which unites all these parent companies under one roof and which will represent the parent root enterprise unit. That major intranet site will probably have only links to intranets of parent companies' and some corporate news, maybe a link to global address book.

Most of these parent companies have their own IT and HR staff so we need to separate management permissions for every intranet website.

What would be the best way to achieve such structure? I was thinking about creating a separate site collection for every intranet website, then delegating management permissions for IT department of different companies. Then create one more separate site collection for root level intranet. What is the best template for such sites? Should I create these site collections through SP Admin, or through "+ Create Site" button? What about subsites for daughter companies, should I just create a sub-site or a separete site collection all together?

Thank you for your answers in advance!

You must be SharePoint Online Admin to see the SP admin center.

the answer to your 2nd question is clearly stated in the message shown on your screen shot. 

In the future, i would recommend starting a new conversation instead of hijacking other threads like this.

Thanks for getting back to me.  Two final questions:

1.  How do you enable the SP Admin Center Preview?  Don't have the banner on the SharePoint admin center.
2. I did not create the new team site, someone else did.  This team site was created under the existing top-level root collection.  I can view the group connected to the new site collection, but I cannot see any owners or members.  I believe it is because I have only Exchange permissions and cannot modify Office 365 groups, yes?

When you do that you are creating a new Site Collection that is connected to an Office Group. It will be created in the path specified in the SPO Admin Center, Setting page, Site Creation-Create Group under option.  This site may not show up in the SPO Admin Center at this time unless you have the SP Admin Center Preview deployed. 

when you create the new site, you assign Owners to it, these people get added to the Office Group where they are assigned the Owner attribute/role and you assign Members, which get added to the same Office Group with the Member attribute flagged. These attributes determine who shows up in the SCA group (office group owners) and the SPO Members group (Office Group members). 

As the SPO Admin, you always have the ability to add additional people to the SP Owner group or the SCA group to any site, but you may need to be granted permission separately to edit the Office Group.

My recommendation to my clients is to learn the new Office Groups, use them and try to stay away from the old SP groups unless absolutely necessary to fulfill a complex security requirement.

Dean,

Lets say I do not want to create a subsite/page off the team site, but go to the SharePoint homepage and click Create site.  I believe I am creating a new site collection not "affiliated" with the Team site (part of the root collection), correct?  Also, what can a SharePoint admin "administer" under such conditions?  If someone creates a new site collection (i.e., SP homepage>Create site), can an admin designate site collection admins?  I am having trouble doing so with Admin privileges.  Also, I cannot view the new team site created by someone in another department, nor can I assign him permissions for the team site he created.  Some of these questions are redudant, I know some of the answers, but just some validation!  Thanks much!

I agree with Dean - the terminology is confusing and often misused for convenience (or maybe laziness as he suggests). As a general practice, I would try to create a separate site collection for each unit of work - a team, a project, or even a department Communication Site. When you create a new "site" from SharePoint home, you are actually creating a new site collection. For now, you can connect "modern" related sites with navigation links or custom web parts. 

It is just terminology. Site collections contain sites (which can also be called subsites for simplicity).

Site collections are frequently called sites, because people are lazy and MS does not have technical editors enforcing consistent usage of terminology in their documentation:)

When writing code, sites are called webs, which makes this whole topic all the more confusing. 

While this article was written for 2013, it applies to SPOnline and  may be helpful, https://technet.microsoft.com/en-us/library/cc263094.aspx

Thanks Dean. I completely agree with the advice of keeping it slow and simple.

A very basic question, I was trying to find out how to create additional "sites" within a collection, so I went to that collection (say https://abc.sharepoint.com/sites/Collection-1) by clicking on the URL. Which is basically the root site created automatically for new collection. On visithing this site, when we go to 'Site Contents' it gives option to create "Sub-sites" (screen below).  I was trying to find out how do we actually create "sites" (instead of subsites) within a collection. If this option of "subsite" at the collection level is actually just a terminology and these are effectively the "sites" then that's fine. Just wanted to make sure I am not missing anything while setting up the base structure.

Truely appreciate so valuable insights.