contributors make REQUESTS but the maintainers do the merging of those requests or reject them. and yes the maintainers can force merge their own code without anyone else. in short it needs the maintainers to decide. they govern bitcoin code

most contributors are not helping with code, but instead just spell checkers or translators. when you look at whom helms the code changes of the big features you see the same short list of names that do the real coding

in short contributors do not have the same power, access, privilege or rank of control as the maintainers
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as for if another brand of a full node has equal chance of making a protocol change vs how easily core changing the protocol is just accepted without any notice/conflict/consent. well in recent years core have taken control and other brands of nodes if changing the protocol are treated as altcoin proposals, not bitcoin proposals. everything these days needs to go through the core maintainers governance to change bitcoins protocol