The standard models are a % of revenue (i.e. Unreal Engine - fine for flat advance services like Game Pass), a flat licensing fee (old Unity, old Fmod, etc), or an ongoing fee per-title/per-seat (current Unity, stuff like Wwise). This new fee-per-install model Unity is adopting is relatively rare.
Game giveaways like Game Pass and Epic Games Store's free games are generally based on a flat advance. I think PlayStation Plus's free games are also a flat advance, but I can't personally attest to it. There are tables out there of how much money Epic handed specific developers for their games and how many copies were installed that I think leaked during the Apple lawsuit, and in some cases developers earned much less than Unity's 20 cents per copy due to the number of installs.
Ideally this flat advance model would have been widely protested by the industry, but there is a huge audience of struggling indie game devs for whom a flat advance is a desperately needed lifeline, so it's been successful.