First off, most of the questions are very opionated (at least the answers are) and your question will probably be closed for that reason. So I will post my answer before completing it and expand on it after.
First off a good roadmap to become a web developer. I like it mainly because it shows the crazyness the web development world has come to (don't be shocked!): https://levelup.gitconnected.com/the-2020-web-developer-roadmap-76503ddfb327
Trying to answer some of your question (answers are my opinion):
Is there some typical way to go / best practice while developing web-applications?
I'm tempted to say there are as many ways to do web development as there are web-developers in the world, but that might be a bit exaggerated. If you want some guidelines, I'd pick one of the major web frameworks and learn the way they do web development. With web frameworks I mean all kinds of frameworks starting with JS-frameworks all the way to static site generators, etc. They all have their ecosystem and their own rules.
What are the typical key players? I know there is the difference between front-end, back-end and databases.
(personal opinion) I work with Go in the backend. I love it because it brings back some simplicity in the crazy world of choices being a web developer. Since you know C, Go will probably be easy for you. It has static typing, structs, etc, but no need to manually manage your memory. It is also much faster than most other backend languages used in web development (Python, NodeJS, PHP, Ruby, etc).
In the front-end I have used native JS, jQuery, React, Vue, etc. I'm still waiting for something that makes things easy again. Flutter seems to be something that has a good approach, but is not really a web front framework (yet). (Don't do public websites with Flutter! They are not indexable.) We'll see where it goes.
Databases I will not go into here as that is another huge topic. Let's just say that I'm more a fan of using multiple databases for their specific strengths rather than a big one that is supposed to be good at anything.
which code is usually written in back-end / server-side languages?
Even this depends largely on your choices (framework and preference). One thing for sure has to be in the backend and that is security related stuff. Anything you put in frontend code is visible to an experienced user.
Apart from that there are some ecosystems where you don't write any backend code but talk to a (cloud) service that is basically like a database with a web endpoint on top with secured login. (for example https://firebase.google.com/.) Here the security related stuff is baked into the service.
If you do both, keeping business logic in the backend is probably a good idea. If the frontend calculates something (for quick response), the backend should double check that (e.g. calculating the total in a cart). But this is too general. There can always be use cases where some business logic needs to be implemented in the front-end.
Do I have to move all business logic into the back-end, also for security reasons or is this maybe also a bad idea because of performance reasons?
Performance can be a problem, but mostly because the roundtrip time to the server and back. If you do that for every tiny information, the UI will become sluggish. You might want to think about doing e.g. a calculation client-side.
JS-Frameworks like React, Vue don't request html from the backend, but data and build the html based on that data client-side. I'd use them if I have a very data driven website / webapp, especially if it is user-dependent. Transferring only the data and building the html for every site from it in the browser based on user settings and data, saves a lot of roundtrips.
If you are worried about server performance: For the server to hit its limit, you'd need heavy usage of your website for that to become an issue (at least with Go). If you get there you can still use horizontal scaling (multiple instances of you server) to solve that. Unless you are working for a large company with millions of users daily, I'd not worry about scaling for now.
What programming languages are more or less dead and not to be used in the future?
Warning: Very opionated!
I'd say PHP is dead. Many headhunters I've spoken with agree with me. Companies are desperately looking for PHP developers, because many developers are moving on from PHP to something "cooler". You'll definitely find a job with PHP, but might not be so happy with your job. For me it is also a sign of how modern a company really is (if PHP is not it's main backend language (any more)).
Python currently has a big boom. Mostly because of AI development. I'm not sure if that boom is also in the web development, but I'd say not. I used Python before Go (5+ years ago) and before that PHP (8+ years ago). I rarely get Python web developer job offers (at least compared to PHP and Go).
Go is the language of the cloud. It is perfect for concurrent programming which is an essential part in web development (every http call should be handled concurrently). It is fast and light weight and doesn't need anything installed on the server to run (compiles to a single binary without dependencies).
NodeJS: Haven't used. I'm not a fan of Javascript (but it was (and kind of still is) the only option in the browser), so I never liked the idea of using it also in the backend.
TypeScript: might be an alternative to JavaScript (thinking of frontend here) if you like a more structured language.
It sounded like you want to build a user baser web app with data being managed by each user. This is what I would (probably) do in that case:
- Backend in Go
- Go serves static files (start html, css, js, images, etc.)
- Go server has an api endpoint that serves data (e.g. REST style)
- Vue (or React) in the frontend
- Vue requests data from the api to build the user-specific content
