"Our sponsors - Mythic Beasts ISP provides us two virtual servers with 400 TB/month of free bandwidth to help our users with maps downloads and updates."

On their github README.md.

Mythic Beasts are absolutely great, on that note. One of the truly old school hosts in the UK. They pay their employees a share bonus, they're technically excellent, and their services are reliable and well priced. I've been using them for years (since Bytemark got eaten by IOMart) and love them.

We pay it out of pocket or from users' donations.

Also consider that with Google you have to redownload your area of the map almost constantly since they're online maps, whereas it's only even possible to download a ~100mb map file from OrganicMaps monthly-ish when they release app updates and most people don't update. 100mb per user is nothing.