Native American English speaker here, learning German. I can explain better since you're German!
Yes and no. :) "Only" is pretty easy to explain, but "just" is a very complicated word. It has several meanings and is used a lot colloquially. It's kind of like "einfach" and "gerade" in German - can be used in similar ways, but they are also different.
"Only" is closest to "nur" in German. It can also sometimes mean "except". That's pretty much all "only" means.
I have only one sandwich.
I'm the only person left in the game.
Please take only four tickers per family.
You may come into the house, only take your shoes off first. (this is the "except" meaning)
"Just" can also mean "nur" sometimes (which I why you're confused about the difference, I assume), but it has several other uses as well. "Just" is used like "einfach", "gerade", and sometimes "eben" too.
You cannot just leave!
I just arrived.
I just can't take it anymore.
That is just right.
Yes, that's just what the problem is!
I have just one sandwich.
You may come into the house, just take your shoes off first.
As you can see with the last two examples, sometimes "just" is interchangeable with "only." Not always though! The last two sentences are the only ones in the "just" examples that you can switch "just" and "only". You cannot with the rest!
So in the end, "only" has pretty much just one meaning, closest to "nur". However, "just" has several meanings, one of which is the same as "nur", but there are many more, like "einfach", "gerade" and "eben."
Read these blog posts for a better explanation. This is my favorite German blog, and even though it's for English-speakers learning German, the posts have a ton of comparison German and English sentences so you can get a good idea of how words are used.
Hope that helps!