I think that you should probably finish the frontend section first.
Would learning node/backend help you understand APIs better? Yes, maybe. But it also might make you even more confused.
I will say that I didn’t truly understand APIs until I did the backend section, and then only after a lot of struggle. But you can get a “good enough for now” understanding. But they are confusing. And unfortunately, they are often poorly documented and change without warning.
Just push forward. And do some google searches. And if you can’t find what you need, ask here. If you ask a narrow and specific question, I’m sure you’ll get a lot of good responses.
Just keep struggling and trying things out. It will come to you slowly.