I understand that Teams leverages SharePoint on the backend for storage and authentication, but it's not really optimal for being used as a true 'shared document repository for large amounts of files.
If I create a Document Library from under the same SharePoint site that a Teams site is using, how will that affect the users' experience using that same site through Teams?
For instance, if I have an HR Teams site, that is associated with the Office 365 Group/SharePoint site 'HR' but also want to have a secured private Document Library under that same SharePoint site, will that new site impact for the users accessing the HR Teams site? Is that document library going to show up under any channels within the HR Team?
Default the only files showing in teams is the files tab which are folders in the root of the default library in the team site!
If you crate another library in this team site in SP you can either add it as a sharepoint tab OR as cloud storage! This is done from any files tab in the team and will end up as a folder in that files tab!
You can set other permissions for this library but all users will se the tab or folder but will get an access denied if they don’t have permissions to it!
No, the separate Document Library is not displayed in the Teams workspace (I usually use this term to differentiate it from the SP site) unless you follow Adam's suggestions.
It has it advantages so that there are not too many libraries created each time a channel is created (admin overhead). Also, this also makes it clear when creating unique permissions if you decide to create another library.
Teams is built on top of SPO as you have well said, so when you create a Team you have a full SPO Site where you can add additional document libraries as required. If you want to show up those document libraries in Teams you have two choices: 1) As mentioned by Adam, by means of a tab (use the Document Library Tab or the Web one) 2) You can add also a document library in the Files Tab by means of the Cloud Storage option In regards of configuring specific security for those new document libraries, you can do it but just remember that secureity in a Teams site is governed by the underlying Office 365 Group so you have to be carefully when configuring custom security...one thing you could do here is just to create a separate Site Collection not connected to your Team and with no Group behind the scenes where you can configure Security as you wish