Hi Daria, Great question. We use both of these expressions to mean that the current situation will get resolved somehow. "It'll" be okay means that the specific situation someone is facing will be resolved. "Everything" will be okay basically means the same thing. Everything that you're worried about will work out somehow. We use them both and in my personal opinion, there's no big difference between the two. I hope that makes sense and is helpful.

Both are usually used interchangeably in conversation, however to get to the specifics of the differences between them, It'll be okay refers to a specific situation or thing, "it," meanwhile everything refers to, well everything. You can say either, they mean the same thing.

Hi