I have one farm on premises with sharepoint 2019 with sites just can be Access by users of AD and just with vpn. Example site: https://mysite.corporativo.pt .Its possible to integrate mysite with teams, and when i turn on my teams on a cellphone or pc without the vpn i can login on my site using office365 account.
I don't think this is an scenario we can have Today with SharePoint OnPrem and SharePoint Online in a hybrid deployment scenario. By the way, take a look at the hybrid scenarios SP OnPrem - SharePoint Online to understand what's possible
