Many common ways for the financing of an free and open source project to be sustainable (The main keyword) in the mid to long term:

1) A non-profit company that has full-time employees but supported via sponsorships: (gnu project, mastodon, omarchy, zig)

2) Multiple for-profit companies that each have full-time employees contributing to an open source project that they all use: (ocaml, linux, rust)

3) An ultra high net worth individual that has full-time contributors to open source and supported via a non-profit (signal, ghostty)

4) A for-profit company that has dedicated full-time employees that have full support and own the direction of the open source project (go, swift, java)

5) VC funded open source projects (Herdr, Bun, Deno, Burno, Atuin)

All other options such as low donations like "buy me a coffee", ko-fi, are completely unsustainable, now that AI agents exist.