Minecraft: How can I install Forge if it always only opens in WinRAR?

In videos like those at 1:20 the program always opens:

Right click and "Open with…"

Yes that opens up too

You have to start this from the command line: java -jar xxxx.jar

With win + r?

OK yes, start cmd.exe there, then change to the directory where the jar file is located, and then use the Java command, of course you have to adapt the file name accordingly. You may have to adjust the PATH variable if Java is not found.

Do you have java installed?

https://www.java.com/...manual.jsp there you can install Java and then open with… Click

I don't know, how do you look it up?

But minecraft java edition works