What's the best way to handle when an employee leaves the company? OneDrive and Email access

You can put the mailbox on hold, then delete the user - this provisions it as Inactive mailbox and will preserve all the data in it: https://docs.microsoft.com/en-us/office365/securitycompliance/create-and-manage-inactive-mailboxes

But they are a bit harder to work with if you need frequent access to the data. Some more considerations here: https://practical365.com/exchange-online/shared-mailboxes-vs-inactive-mailboxes-departed-users/