Ho hum. Laws are often hierarchical.
So in 1948, the UN gave the world the Universal Declaration of Human Rights. Nice idea, not necessarily legally enforceable on it's members or signatories, but eventually transposed into national and international law. But because of multiple legislation, conflicts and case law also feed in.
Then there's Blighty, Brexit and Brussels. So EU law does apply, as does national & international law. So there are transitional arrangements, where UK's agreed to follow existing EU diktat. For some reason, that's temporary, and the UK didn't agree to being permanently bound by EU law without representation.
So suppose I'm a migrant who's survived the trip from Calais to Kent. On arrival, I utter the incantation given to me by my travel consultant. I wish to claim asylum in the United Kingdom. Legislation kicks in, and I'm a protected person. Then maybe the Border Force checks my prints and points out this is my 3rd attempt, and 3rd identity. Now I'm an illegal immigrant.
Then the UK could return me to my point of entry for deportation. France says 'Non!'. But France is still in the EU, bound by EU and international laws and treaties, and arguably responsible for that migrant. And then France could use Dublin III to return me to the EU point of entry, and deportation becomes that Member's problem.
So again, it's a wicked problem. It's also a very serious issue. DM ran a story about two Ukrainians who were raped by other Ukrainian refugees in Dusseldorf. Lede seemed odd because it says 'She was allegedly attacked by two Ukrainians, 37 and 26 from Iraq and Nigeria' and me thinking 'that's 4 men'. The male victim was allegedly drugged and raped by 2 Ukrainian-Morroccans.
Needless to say, the German authorities are distinctly unhappy. But there's also the question of what to do next. Obviously the rapists are criminals, and in the process of being prosecuted. Ukraine could , and arguably should be stripped of their Ukrainian identity, which could lead the rapists to being deported back to their original country. Or the Morroccan rapists might appeal deportation because they might be persecuted for being gay. And lawers will get richer defending the indefensible.
But such is politics. Plus it's been a concern pre-war around Ukraine's EU accession given Ukraine's been a bit generous in handing out (or selling) citizenship.