Don't services have full access to the emails?

E.g. Protonmail (insert any privacy respecting service) I can login from any device and fetch my emails. Don't they need to have my password as well to check that it's the right password? And if they have the password, can't they fetch all emails themselves? This applies of course to all other services that claim that only I have access to it.

Sorry, if this is a noob question but I can't get my head around it.

access don emails privacy services