You can compare it to https://apt.syncthing.net/ which is “official” and you will see it is pretty much the same. The further steps are either optional (making web UI publicly available), configuration or outdated (systemd services aren’t missing). As to the difference to just downloading: It’s the general difference between running a standalone binary and installing through your distributions package manager.
Again: outdated. When in doubt (that’s neither Syncthing nor software specific): Go with the original source (i.e. stable).
Further down it does something with git, so it needs to install git first. No need to do that though.
Distribution: System you installed (@everyonewhoknowsbetter please don’t kill me for this). Usually those who provide this distribution, also provide a tool (manager, in your case apt-get) to install “curated” software (packages).
I hate to say - But I dont know what user I am logged in as. I just install the Raspbian OS - I got no log-in procedure. Now I just want to install Syncthing the right way - also it could be fine if I could figure it out to start up with OS (when restarting and so on)
You tell me - and I am very thankfully - its best to do the way as described in your link.
Now i ask you (and you know that I have install the syncthing by the way it was described in the link, I showed):
If I now install after the procedure from apt.syncthing.net, Will I keep my current profile, settings and the Folders that I already have set to sync?
First rule of lazy administration: If it isn’t broken, don’t change it. So if it works currently, no need to change anything.
I answered the question about whether the config/settings/whatever are conserved: Yes if you start syncthing as the same user.
As to all the rest of these command, e.g. what your user is (run whoami): That’s just googleable. I am very happy to answer specific questions that you couldn’t find an answer to, but I didn’t get the impression of too much research and I don’t have unlimited time - please excuse the frankness.
Just remember - What seems obvious logical to you (like walking) because you may have worked with that linux logic for many years can be strange science for someone who takes his first steps
In fact - I try to care about the almost universal fact that your and all people’s time is limited by trying to make questions that you could just answering yes or no to (You are not that kind of type ;).
It would also be a waste of time if all that you (and I) have wroted did not end up with that the problem / the question was solved / answered and that the answer was understood by me