Right so WhatsApp and messenger are gatekeepers and they must allow interoperation with who anyone who wants to ie me running my own signal instance?

There are several stipulations on interoperability in the new regulation (Ctrl+F "interop"). To my understanding it is stipulated that they have to make interoperability possible for certain third parties, but how to go about this is not exactly specified on a technical level - meaning the specific way to implement this is left to the gatekeeper. So your Signal server may or may not be able to depending on how exactly they go about this.

They also need to interoperate with signal hence if a works with b and c works with a why wouldn’t b work with c?

No they need to enable interoperability period. Says nothing about Signal (the software) per se. Meta has announced they plan on implementing it based on the Signal protocol (not Signal messenger software, not Signal server software).

Cos if thats hoe it works or if im not allowed to interoperate with WhatsApp or messenger in the first place then this juat seems like its handing the monopoly away from the companies to the government and giving the people fuck all.

To my knowledge the aim of the regulation is exactly that, to allow anybody interoperability with these "core platform services". The status quo is that the regulations has been announced by the EU, it has gone into effect, and Meta has announced how they will implement interoperability to comply. Once the implementation is available and then found lacking in regard to the regulation it would be up to the affected third party to sue Meta over it.