Scriptmail: technical help requried.

I'm student of information security, currently using proton email and i need to clear some questions before buying this service. please explain me following questions.

Q1. Is PKI Keys especially user's private keys are generated on server or on client side?

Q 2. Is user's private keys are saved on client side or on server side? for example proton email saves user's private key encrypted with mailbox password at their server.

Q3. Can you provide complete working diagram or text explaining how automatic key management works.
For example in proton:

User creates key pairs at client side, and public key and encrypts private key with mail-box password is transmitted to server. mail-box password in never transmitted to their server.

Working: (follow numbering in attach image)



I need similar explanation...

Q4. How meta data is on script mail protected?