The manufacturer is often referred to as a third-party manufacturer when offering its services on third-party platforms or environments.
The term sounds a little unsettling, but most manufacturers, dealers, providers act as so-called third-party manufacturers or third-best!
Simple example: Your PC is from Dell, so MS would be a third party since it runs on Dell's PC and is not Dell's own OS. If you now install software, drivers, etc. on MS OS, it is mostly so-called third-party manufacturers or third-best!
The third party term is very broad but is nothing threatening because most of the offers, software, and... are third party solutions or products!
Why does third-party manufacturers, foreign workers, third-party providers, ... sound unsettling to us?
It's because it's not number one, who wants to be second best or third best schnitzel at a restaurant, or last in line. As a human being you always want the best for yourself and your friends, family, ... for this reason the three is not instinctively good enough for us, even if a bronze medal at the Olympic Games is a very good result!