Legality Of API's

Hey Indiehackers,

I'm currently in talks with several web devs (and teams), looking to build an MVP for a SaaS business.

It would cost a pretty penny to build the whole thing from scratch, but I think it's a lot better to use API's for an MVP, get paying customers then build our own solution from there.

However, I'm not 100% sure how the legality of API's works or where to even find the necessary API's for the solutions I need.

Does anyone here have expertise in this field, or know about the legality or where to find API's?

Thanks