SimpleLogin is pretty much owned by Proton, so you can create an alias form SimpleLogin’s Web UI/mobile app or from within ProtonPass, it will end up being the same.
Feel free to read the recommendation page to understand a bit better the details. 
PS: and be careful of not confusing Email accounts’ aliases vs Email aliases as explained here.
The first one is just an alternative name to access your account while the second is a proxy that adds anonymity/convenience. Hella confusing I do agree. 
Btw, feel free to check some of those previous questions here:
- Email Aliasing Strategy Planning
- Overwhelmed newbie - rushed email strategy guidance
- Checking email from addresses
Plenty of answers already available there. 
Given a read to all the links above then feel free to narrow down your question a bit if you want some further help.