Hey Marc.
There are free learning paths from Microsoft for a lot of 365 things, from Exchange to Teams. A good starting point is their MS-100 exam criteria. Even if you don't sit the exam, the learning material is all fundamental stuff you'll need for 365 administration.
https://docs.microsoft.com/en-us/learn/certifications/exams/ms-100Towards the bottom of that page, you'll find a list of Learning Paths covering things like Azure AD (the backbone of managing users, devices, and apps).
You can also get a free M365 E5 licensense as a developer for labbing stuff out from here:
https://developer.microsoft.com/en-us/microsoft-365/dev-programIf reading through the Microsoft Learning Paths isn't your thing, there are good video training for the MS-100 on sites like Pluralsight which will also have a free trial.
Then when you've completed the MS-100 path, onto MS-101
🙂Also might be worth checking out the following (paid) eBooks for some real technical depth on M365:
Office 365 for IT Pros (general Office 365):
https://gumroad.com/l/O365ITMicrosoft 365 Security for IT Pros (security-specific):
https://gumroad.com/l/mdGbp