You've already gotten a lot of good replies, but I was in your exact position and I want to share another perspective of what worked specifically for me.
First and foremost, stop worrying about making a "real application". Don't concern yourself with making this something that people will actually use, or something that could potentially make money, or the next Facebook. Just stop. You're never going to actually build anything if you keep second guessing whether or not it's a good thing to build. Instead, just acknowledge that your goal is to build something and to learn from it and have fun building it. If it turns out that your app becomes "real", then that is a huge bonus! But focusing on that as a priority is just going to be a blocker for you.
Second, if you are in a position where you are trying to "learn" new skills, then I would actually advise against starting with trying to build your Spotify/MapBox idea. Instead, pick an already existing something that either a) you already know a lot about or b) is a common app that people build (like a blog site or a reddit/HN clone). I know that sounds frustrating and not ideal, but I went through the same ordeal and it helped a lot. The reason for this is that there are tons of good tutorials for building something like a blog or social network site with all kinds of tech stacks, so that will help. The other reason is because you already have a feature roadmap laid out for you, and this will help get past the analysis paralysis.
For example, you can build an HN clone and just start off small by building the front page which links to other news sites. Then you add a login system. Then add comments. Then add points and ranking. Each step is small enough to actually be feasible, but probably tough enough that it will teach you new aspects (such as teaching you how to do logins correctly, or how to store comments effectively in a database, etc). And at each step, you still have the opportunity to add your own improvements as you see fit, so even though you are essentially building an HN clone, you can still add your own flair to it and explore your own personal interests.
I did the above, and I found that it was immensely helpful in actually getting past the analysis paralysis stage that it sounds like you are stuck at. After I built a simple news aggregator clone, I found that the process had taught me enough to then actually build one of my own personal ideas, and because I now had the foundational knowledge from building the earlier 'simpler' app, it was much easier to get a jump start on my own app.
Just wanted to second the recommendation to build something that already exists first. Like how artists do master studies, or still life, software developers can learn a lot by actually building something that already exists. It helps educate you about the nature of how certain things are built, and what considerations they have.
My first web project in Go was an email signup server. The second was a blog engine that still powers https://junglecoder.com. Both projects taught me a lot, and the blog took quite a while to finish up.
Getting something out the door and in front of other people, even if it's just your friends, is key.
First and foremost, stop worrying about making a "real application". Don't concern yourself with making this something that people will actually use, or something that could potentially make money, or the next Facebook. Just stop. You're never going to actually build anything if you keep second guessing whether or not it's a good thing to build. Instead, just acknowledge that your goal is to build something and to learn from it and have fun building it. If it turns out that your app becomes "real", then that is a huge bonus! But focusing on that as a priority is just going to be a blocker for you.
Second, if you are in a position where you are trying to "learn" new skills, then I would actually advise against starting with trying to build your Spotify/MapBox idea. Instead, pick an already existing something that either a) you already know a lot about or b) is a common app that people build (like a blog site or a reddit/HN clone). I know that sounds frustrating and not ideal, but I went through the same ordeal and it helped a lot. The reason for this is that there are tons of good tutorials for building something like a blog or social network site with all kinds of tech stacks, so that will help. The other reason is because you already have a feature roadmap laid out for you, and this will help get past the analysis paralysis.
For example, you can build an HN clone and just start off small by building the front page which links to other news sites. Then you add a login system. Then add comments. Then add points and ranking. Each step is small enough to actually be feasible, but probably tough enough that it will teach you new aspects (such as teaching you how to do logins correctly, or how to store comments effectively in a database, etc). And at each step, you still have the opportunity to add your own improvements as you see fit, so even though you are essentially building an HN clone, you can still add your own flair to it and explore your own personal interests.
I did the above, and I found that it was immensely helpful in actually getting past the analysis paralysis stage that it sounds like you are stuck at. After I built a simple news aggregator clone, I found that the process had taught me enough to then actually build one of my own personal ideas, and because I now had the foundational knowledge from building the earlier 'simpler' app, it was much easier to get a jump start on my own app.