The "rules based international order" is supposedly the body of international laws and customs as well as their enforcement internationally, and is what the US uses to justify their hegemony, placing them as the guarantor of this order - while themselves violently breaching it.
It's also called the "US-led international order" or the "liberal international order". Put simply, it's the US and its allies enforcing a set of rules internationally with violence or the threat of violence (or economic sanctions)
See: https://en.m.wikipedia.org/wiki/Liberal_international_order
Also Somalia originally wanted to split in multiple countries and NATO interventions happened to prevent that and force them to remain only one country, and remain in Civil war
How the EU member states act towards parties outside the EU seems a bit beside the main point: that the EU itself actually is about international rules and not a cover for violent actions between the various member states.
The "rules based international order", on the other hand, is not voluntary. If the US decides that you are in breach, then it will be rectified with hard power. It's simply an abstraction for hegemony, and you don't get a choice to parcipate or not, the choice is made for you by the hegemon (which also happens to be its enforcer).