If I donate XMR to e.g. the developers of a free software project, how are taxes handled? Is the developer assumed to report his profits to the state and handle the taxes or do most devs just keep 100% of the donation?
If it would somehow be made public that I donated x amount to a dev and that neither of us payed taxes, would the dev be taken accountable or me or both of us?
This is correct, the state has no jurisdiction over Monero. Once you use their toxic fiat system that is when they hook into you.
Of course they will try to trick you into thinking they own you and everything you do, this is the devilish deception and tactics they use as they have a firm grip on most people via sophisticated fear based mind control.