SharePoint Site Collections
Site collections in SharePoint are something that can be a point of contention between SharePoint afficianados.
Site collections are basically a big basket that contain all sites under a single URL.
One point to make that is pretty key here is that in on-premises versions of SharePoint, site collections have their own content database.
In all other forms, including Office 365, SharePoint Site Collections are basically defined by a URL. So your (sub)site’s URL would be https://[YOUR COMPANY].sharepoint.com/sites/SiteCollection/[Site]/[Subsite]. Using This helps to keep things separate and organized.
Now a days, most of us in the SharePoint world agree, you should avoid using sub sites for anything besides very specific use cases like a short-term goal or project. (but even then, use Teams for that!)
Reason being that subsites inherit permissions, and also Permission groups from their parent site. Permission groups should be defined at the parents site. Also, site collection administrators have full access to files and content in the site they administrate. This means if you have a user with SCA access, they would be able to access HR files, or other sensitive data. The user may not care and be trust worthy, but it’s better to prevent that possibility from ever becoming a reality. Also, this makes searching and content search webparts easier to setup and manage because you can simply set it up to search only within the current site collection.
If you’re looking to setup a Main company site with other sites for departments, you should create a site collection for each department.
Something like this:
Top level site
- https://[YOUR COMPANY].sharepoint.com/
Department Site Collections
- https://[YOUR COMPANY].sharepoint.com/sites/BILLING
- https://[YOUR COMPANY].sharepoint.com/sites/SALES
- https://[YOUR COMPANY].sharepoint.com/sites/HR
- https://[YOUR COMPANY].sharepoint.com/sites/IT
- https://[YOUR COMPANY].sharepoint.com/sites/Marketing
Each site collection will allow you to designate different Site Collection Adminstrators, separate sharepoint/security groups, and content search will function better as well.
This way you’ll not only be able to have clearly defined urls, but you’ll also compartmentalize security and in general have a better time orchestrating and administrating SharePoint.