If it’s free, how do the companies making it stay in business? Not all Linux variants, or distributions/distros (there are a lot), are backed up by a company. The ones that are offer an enterprise version (like Microsoft sells Windows Server) for a price. Since “Linux” refers to the core of the OS, and its license dictates that it must be offered for free, you can get these enterprise versions for free. They charge for updates, levels of support, and some in-house developed solutions that work well with their distributed.
If the source code is open and maintained by a community for free, how is that better that something made by professionals at a company? This is a huge topic, but to boil it down: First, the projects responsible for Linux distros and most of the utilities found within are run quite competently. It’s quite difficult for some random person to insert janky code. Second, most of the people contributing to these projects are often professional coders in their “day job,” or their “day job” includes contributing & maintaining this project. Third, when some random person finds a bug or exploit in these utilities and operating systems, the process for reporting and resolving it is pretty transparent compared to what you’ll find with for-profit software companies. Because the code isn’t proprietary, processes involved in auditing it tend to be open to the public.