How can an open source company have a valuation of $1B?

How do open source businesses work at all? I sometimes read about companies like these, with huge valuations and then the next day I read about some open source developer asking for donations because their project, which powers half of the internet [1], is not making him enough to pay for food. What makes the difference?

Over the years, I wrote some tools for my own use, which are way better than their commercial counterparts. If I would spend a few weeks to polish one of them and open source it, it would probably gain widespread adoption. But how could I then build that into a business?

[1] Relevant XKCD: https://xkcd.com/2347/