Below are a few training sites and videos to help you with the migration or use Office 365 applications.
Learn by particular app or service:
Interested in training on a particular application?
https://support.office.com/en-us/article/Office-Training-Center-b8f02f81-ec85-4493-a39b-4c48e6bc4bfb
Quick start guides:
Downloadable Quick Start Guides & Reference Cards
https://support.office.com/en-us/article/Office-Quick-Start-Guides-004185ad-d0fe-4046-85a4-5204b8b4989b
What's new in Office 2021:
What's new and improved in Office 20
https://support.microsoft.com/en-us/office/what-s-new-in-office-2021-43848c29-665d-4b1b-bc12-acd2bfb3910a
Switching from guides:
If you're use to a different set of tools or an earlier version of Office, use one of these guides to help you quickly find your way around Office 365
https://support.office.com/en-us/article/Switching-to-Office-365-39d3f38e-67af-492f-a11a-10e07ee65ec7