They don’t. Libre Office is maintained by a non-profit called The Document Foundation. They’re funded entirely by donations. I think they make enough to have some full time employees.

A lot of open source software is created by individuals or non-profits. The Mozilla foundation makes Firefox, for instance. They make money through donations and also Google pays them a ton of money to be the default search engine.

There are for profit companies that make open source software. Such as Red Hat. They tend to make money by selling support for the software.