Turning on Modern Authentication with mixed Outlook versions

An ideal scenario would be that we enable it for the whole organization and nothing would change for all accounts that are already configured.

We would then let most of it phase out:

-> New users would get Modern Auth

-> Existing users receiving new device would get Modern Auth

-> We would switch others in batches

This would allow us to do it at our own pace without causing a big bang by enabling it for the whole organization.
However, I couldn't find any official article that explained this in detail.

Nowhere is mentioned what would happen to existing clients and older Outlook versions.

And yes, we don't like app password either. Modern Auth would allow us to get rid of it.