https://serverfault.com/questions/24523/meaning-of-directories-on-unix-and-unix-like-systems

• /bin - Binaries.
• /boot - Files required for booting.
• /dev - Device files.
• /etc - Et cetera. The name is inherited from the earliest Unixes, which is when it became the spot to put config-files.
• /home - Where home directories are kept.
• /lib - Where code libraries are kept.
• /media - A more modern directory, but where removable media gets mounted.
• /mnt - Where temporary file-systems are mounted.
• /opt - Where optional add-on software is installed. This is discrete from /usr/local/ for reasons I'll get to later.
• /run - Where runtime variable data is kept.
• /sbin - Where super-binaries are stored. These usually only work with root.
• /srv - Stands for "serve". This directory is intended for static files that are served out. /srv/http would be for static websites, /srv/ftp for an FTP server.
• /tmp - Where temporary files may be stored.
• /usr - Another directory inherited from the Unixes of old, it stands for "UNIX System Resources". It does not stand for "user" (see the Debian Wiki). This directory should be sharable between hosts, and can be NFS mounted to multiple hosts safely. It can be mounted read-only safely.
• /var - Another directory inherited from the Unixes of old, it stands for "variable". This is where system data that varies may be stored. Such things as spool and cache directories may be located here. If a program needs to write to the local file-system and isn't serving that data to someone directly, it'll go here.

Don't delete it. It's an area of the filesystem where the current user session data is kept. This includes things like sockets to communicate with other session components and lock files. It's usually hosted on a ram disk so takes up no space in the system and goes away when you shutdown your machine.

I'd say a good rule of thumb for a beginner is not to touch anything outside of their own home directory. Modifying or deleting files in other locations is an easy way to break your system.

Then I got a lot of duplicate files. For example in subfolders relating to my mp3 player I even discovered my whole NextCloud 'drive' is there again: /run/user/1000/doc/by-app/org.strawberrymusicplayer.strawberry/51b78f5c/N

In Linux a file can show up several times in the filesystem without being duplicated. Symbolic links and hard links will cause this to happen, and they're a normal part of organizing the filesystem. Just because you see a file in several places, that doesn't mean disk space is wasted with duplicates. There may be only one physical copy of the file, appearing in multiple places. With hard links you need to be especially careful about deleting, since you'll think you're deleting one of several duplicates but you'll in fact delete the only copy of the file.

Never touch anything in /run directory.

Something to realise when starting with Linux is that everything is a ‘file’. Sockets, processes, input, output etc. That’s very different from Windows and part of why scripting on Linux is so powerful. You can interact with anything.
So some directories are filled with things that aren’t necessarily files but look like it. Someone else posted a whole list, just realise that under those directories/paths shouldn’t be messed with unless you know what it’s for.
Generally when you’re getting used to Linux, /home/$user (aka ~) is where you put personal things. The rest is managed by OS and applications, don’t worry about it.
Edit: spelling

/run contains all sorts of virtual stuff, it doesn't persist over a reboot,

I would advise against deleting anything in it as those files are used by programs running as whether user has the ID of 1000 (most likely you)

it contains things such as sockets and lock files so that programs can interact with each other