Hey Leho.
There are 2 differences here:
> as a Partner that is performing Administrative tasks on the environment (configure security, restore environments, etc), you don't need a license. You just need delegated admin privileges as dicussed on learn.microsoft.com/.../for-partners-delegated-administrator.
> If you need access to the Data, then two important things are required: First, the EndCustomer should NOT create a guest user account for the partner. And second, once the Delegated Admin is granted, the EndCustomer should assign a License (sales enterprise, Sales Professional, etc) to the DelegatedAdmin account. This will allow the access to the data.
Please note that Delegated Admins have certain restrictions (like they can't access make.powerapps.com). An alternative, would be that the EndCustomer generates an account (let's say "PartnerA@EndCustomer.com") that is going to be used by the Partner. In this scenario, despite the account is for the partner, it's a regular user that will require a license.
Regards,
