Is it possible to apply mailbox permissions for members of a group?

Hope you are all healthy!

We're migrating from onprem to 365 and are currently hybrid. We've taken the opportunity to clean up roles and permissions using the "person>user group>permissions group>resource" model.

For most things this is fine, but I'm scratching my head on how to set delegation permissions for a role rather than a person/mailbox.

For example, UserA is the Sales Manager and UserB is the Sales Manager's assistant. 

UserA>SalesManagers>SalesFullAccess>folders/sharepoint etc

UserB>SalesManagerAssistants>SalesManagersMailboxReadManage>all sales manager mailboxes

It's that last step I can't fathom. If UserC becomes a Sales Manager, they'd be added to the SalesManagers group but how would UserB get access to UserC's mailbox?

Is this a PowerApps thing? A PowerAutomate thing? A PowerShell thing?

Apologies if this should be in Exchange, but it feels more like a GROUPS question to me.