Modern vs Old style Site Collections: what's the difference?

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Are there any information on the internet where I could find 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 and put links into navigation from all other sites. What is the best template for such sites? Should I create "modern" communication sites for this, or better "legacy" style intranet 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 separate site collection all together?
Thank you for your answers in advance!

1 Reply
My two (or more) 2 quick cents here:
(1) For now, you can only create classic SPO Sites from the SPO Admin the new upcoming SPO Admin Center (Today in preview in Targeted Release) you will be able to create classic and modern team sites
(2) Classic and modern team sites are not interchangeable. A classic SPO site is based on SharePoint template and a modern SPO site on another one...but in both kind of sites, you can have modern features such as modern lists, modern document libraries and modern pages,
(3) When you create a subsite, for now you have to choose a classic spo template...there are not modern sites templates for subsites
(4) You can extend both classic and modern spo sites with any additional stuff you need (libraries, lists, site columns, content types, etc) by means of PnP or Site Scripts / Site Designs