The kernel and many of its components are. You can download them legally and use them without paying anyone

A different issue would be maintenance for companies. Some companies offer you a service with linux and maintenance, and this is paid because it involves a specific job.

If you know how to use it you don't have to pay anyone.

However it must be stressed that Linux is the kernel, then on top of this there are other components that many are also free as the graphical environment. Whether it is gnome, kde, xfde and others