In each of those cases there's absolutely a process (albeit a long and arduous process) for secession. It's a process truly democratic countries need to have.

Don't forget that Scotland very nearly did leave the UK, with the government's consent, in 2014 (but the referendum didn't pass).

The Catalonia case is a bit murkier, since the Catalan government did not actually reach an agreement with the Spanish government before they went ahead with the referendum. Also because of irregularities seen by international observers during the referendum, the Catalonia referendum result was generally not recognized.

I don't think the USA has a legal process for secession.

Yeah, there's at least a supreme court precedent, set by Texas v. White. It outlawed unilateral succession, but ruled that "revolution or consent of the states" can lead to secession.

The whole point of secession, whether in the US, or Catalonia, or in China[1], is that you have reached a point where you are not able to come to a bilateral compromise on issues that affect you.

If you need bilateral support to exercise your right to self-determination, you don't actually, in practice, have any right to self-determination.

It's like saying that you have the freedom to leave your abusive job, but only as long as your boss lets you quit. That's not freedom, that's slavery.

[1] And yes, I am aware that the case in Scotland was a bit different. Props to the UK for being, as an outside observer, reasonable about the whole affair.