From the 2 developers and The volunteers… The same can be asked about a lot of foss software. Typically what stabilizes foss development though is when developers start getting paid to contribute to the project by a company they work for, however lots of foss software has made it purely through donations (easiest example being mediawiki and wikipedia)
Web hosting is definitely the harder question. In the grand scheme of things, lemmy instances and other fediverse tech will likely end up being pseudo-centralized with a handful of companies like email. Lemmy is very resource intensive as you guessed. The good news is that a very large amount of that resource consumption is storage, and storage is cheap. Though I know I’ve seen tehdude, the owner of the sh.itjust.works instance, another very stable one, comment on how CPU, networking and memory intensive a busy instance can get. A lot of the early 500s instances were seeing were definitely caused by resource constraints.