Each user automatically gets a personal SharePoint “My Site” with a Document Library specially built for OneDrive for Business. They see this as the “OneDrive” web app.
So I create a new mailbox, sign in to OWA and click on the OneDrive app. This is a link to a SharePoint My Site, their personal site
a link to a SharePoint My Site, their personal site with a Document Library specially built for OneDrive for Business
You can use it to sync files offline in your SharePoint “My Site”, which is your own site in SharePoint which also has a Document Library. This “My Site” is what appears when you open the “OneDrive” web app.
Each Document Library can have up to 30,000,000 documents. ODFB can sync 20,000 files offline from your “My Site”, and 5,000 files from any other Document Library that you sync using ODFB tool.
Backups are performed every 12 hours and retained for 14 days. Restore from backup can only be done at a Site Collection level in most cases.
By default, the Team Site Members group has Edit permission, and the members of this group are “Everyone except external users” - far too generous! Delete it! But don't lock yourself out. Make sure to add yourself to Team Site Members and Team Site Owners firstThink of a better permission structure as an immediate priority, before people start messing with it.
Also, adding a permission to a sub-site seems to grant permissions across the entire site collection. There may be a workaround here - 2013 - Can I have subsites inherit permissions from the top node but also add subsite specific permissions without breaking inheritance? - SharePoint Stack Exchange