If you installed the open source that means you will have to build Qt?

Note the question mark at the end, indicating that I was uncertain that is what they wanted/needed? But I understand now :)

How do I build Qt from the files that came from the installer?

That question came after you wrote that Qt needs to be built :-)

I think OP means he installed Qt binaries licensed as OSS, not commercial

But if OP was asking about binaries why their

How do I build Qt from the files that came from the installer?

? If they just want a working system on their Fedora 40 I would have thought installing the distro's version via apt-get or similar would be by far the easiest.....

If you installed the open source that means you will have to build Qt?

I think OP means he installed Qt binaries licensed as OSS, not commercial

There's no reason for the online installer to alter your system such as changing the PATH environment variable.

If you want to develop from the command line use the complete path to qmake or the qt-cmake wrapper. That way you stay in exact control of what you are using for development. This is even more valid when you have several versions of Qt installed.

As for getting started, just use Qt Creator and create a new project through it. You'll have a skeleton application that you can then continue to develop.