Angel Investing - Help me understand

I have an MVP version of a startup idea. No revenue yet. Someone is keen to angel invest. How does this work? Can someone answer these questions for me?

• If suppose Person X puts in 50L, how do I value my company and thereby equity?
• How do I authorise and distribute shares?
• Someone mentioned I should do a SAFE - can someone explain how'd that work and would that be a better option than choosing a random number
• If I give 20% of company for 50L, how does that change if I raise a future round? Do I issue additional shares?

Anyone who has had experience here will help