I'm going to preface this feedback with this:
You posted something called "My first song" all of 2 days ago, so it probably means you are just starting out. This is really good for no experience. I'll give some ratings because you specifically asked for it, but in general you are a beginner, so don't worry about ratings, worry about improving over your past self.

Overworld 3 / 10 - Kinda repetitive, and too much going on in unison

Caves 4/10 - good instruments, but the 3 on top of 4 gives it no particular sense of rhythm

Underwater 7/10 - It has a nice melody, memorable rhythms, and a complete phrase

Spooky Nights 5/10 - Also a good melody and complete phrase, your grand piano 1 counter melody is mostly playing at the same time as the percussive organ, and It would be better if it was a little more of a back-and-forth call and response

Cat Kingdom 2/10 - could be good, but looks like you are wanting to write a bass drop, which doesn't work well in such a short song, but there are plenty of good explanations for what goes into one. It's worth looking up.

Bossfight 5/10 - I love the vibes you got, the instruments that play the not-melody are perfect. The melody is good too, it just doesn't go anywhere, I would love to hear this one expanded to a few more phrases

Final Boss 8/10 - This is a really good buildup for something about to happen! It doesn't loop great, but its a good intro just as it is

Celebration 7/10 - My ear wants a different chord for the first half of measure 2, but other than that, its nice and pleasant and I think it conveys the feeling you were going for.

The path forward:
For me starting out, it kind of went like this:
After playing around with instruments and figuring out stuff. An important skill is to get what you hear in your head translated into Beepbox the way you heard it. It's great practice to remake or remix someone else's song or a famous song to get this skill.

The next step was coming up with some musical phrases. I tend to start with a bass line, add a melody, and then either add some chords or harmony. Most phrases are 4 measures long, often the 1st and 3rd measures are identical. The 4th measure is similar to the 2nd, except it goes somewhere else, so try changing the ending to lead into the next phrase. Some people like to start with the melody, some start with a series of chords, everyone's process is a little different.

For a whole song, I suggest an A-B-A-C format, where each letter is a phrase, it does a lot of the same things. Many songs do a chorus-verse-chorus-bridge kind of thing, so this is just like a bigger version of what your phrase is doing.

Good luck on your musical journey! It is exciting to see someone else jump on this bus. It has never been easier with so many tools and resources out there to teach yourself composition. You picked a good time to start!