snapcraft VS com.spotify.Client

Hopping from one distro to another with a different package manager might require some time to adapt. Using a package manager that can be installed on most distro is one way to help you get to work faster. Flatpak is one of them; other alternative are Snap, Nix or Homebrew. Flatpak is a good starter, and if you have a bunch of free time, I suggest trying Nix.

My personal favourite IDE for java is Intellej Idea. Apart from not demanding the extra extension, It was designed special for Java and Java related languages so it runs java smoothly with great compilation time. so lets install it. Make sure you have snap before installing it

Linux Mints App Store is full of GUI programs, Snap Store ist full of it, Flathub is full of it.

You are being lazy. But I recommend bringing your ass directly to snapcraft.io and reading those documents in the Learn section!!

https://github.com/snapcore/snapcraft - not sure, but looks like a building tools for snaps

Besides, there may be other ways to install them, although there doesn't seem no such Flatpak packages in Flathub. For example, some senerio to use some release channel or Docker / Podman. Additionally, when you use a different Linux distro where systemd is adopted and therefore can do Snaps (Snapd), you have another possibility.

Devuan GNU+Linux is a fork of Debian without systemd and hence Snaps (Snapd). Even on it, it's easy to install Android Studio and start to develop Android mobile apps.

There is nothing "unsafe" about flatpak. In fact for every package on flathub you can see exactly what the "unofficial" packagers do in the corresponding github repository. For example here is the repository for Spotify, if you look at the json manifest file you can see they are repackaging the "official" Snap with the necessary libraries and scripts to support it running as a flatpak. Everything there is open source and able to be examined.

If you click on the "Manifest" link for the package on flathub, it will take you to where the code that packages the app resides. There you can review exactly what the packager does in the json manifest. Here's the one for the Spotify client, for example, which shows where it pulls in dependencies and where it downloads the proprietary code from (in this case they base it on the snap) and what commands it executes while packaging the flatpak.

Try installing it via Flatpak

Flatpaks builds are made to be reproducible. Everything is done openly and you can check how the apps like for example Spotify where made.

Why not just use the flatpak?

Judging by the manifest, the Spotify Flatpak is built from the Spotify Snap.