How do I change my schema without data loss in MySQL ?

Hey everyone, I'm pretty new to SQL so I'm having some disturbing questions in my mind.

How should I proceed when I need to add a new and notNull column to a schema without losing data?
Do I need to set a default ? But then I can't remove the default because it causes data-loss, so I might aswell not use a notNull field, but what if I have to ?

I've been reading about migrations, but I don't really see a big difference between a migration and a db:push (I'm using drizzle + planetscale)

What is the correct way to approach this problem?

Thank you!
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