What is it encrypted by? Or asked differently, what does the user need to have to decrypt it on all their devices? And how can we know that Apple & Co. don't al

What is it encrypted by? Or asked differently, what does the user need to have to decrypt it on all their devices? And how can we know that Apple & Co. don't also have access to that? It's not like Law enforcement hacks into your connection, they just force the company to hand out your data.