The actually free crypto wallets have teams that rely on their main job and donations. Believe it or not, people who do this for everyone else and not for personal gains still exist. They’re rarities, but they exist.
As you may expect, donations usually aren’t considerable and they’re more like tipping.
Donation mean they don't consider it as their main income. They work like a habit to serve what they like to do and donations can be small or big, sometimes donation is big but they don't expect big donation to keep their projects running.
That’s why they have to rely on other sources of income. I’d guess there are also a few devs who have enough money and passive income to not give a shit about being paid to do fun development stuff.
Donations are popular with open source projects like you can get list of Bitcoin Core developers and their donation addresses.
Bitcoin Donation PortalSupport bitcoin developers so they can focus on building our future