And the US is a federation, whereas the Eu is something of a confederation. Both consist of a number of states. So how does for example moving between states in the USA relate to immigrating from outside the states? Would it be discrimiation if individual states i the US accepted more immigrants from other US states that from outside the US?
Well technicly Birtain is afaik a "royal union"? It is atleast a union of somewhat distinct cultural groups rather than centered around being the nation state of a certain nationalistic identity. (it shows during Brexit i must say) It's not like say Norway which is pretty much exclusivly inhabited by Norwegians.
Euh i feel you are heading into a field of semantics where you even have somewhat ill defined what the UK is within the perspective of nation states as to make an in conceived argument how states relate to con/federated unions thereof for what regards internal and external migration. I disagree both with the idea of the UK being the typical nation state and that distinction between internal and external immigration to be something usefully described as discrimination rather than simply "egoistic nationalistic interrests".
The problem as it is is that "the practicle reality NOW is that FoM applies". Meaning that in practice the EU serves as a confederation of states in that regard.
I accept the confederalist nature of the EU and as such that having a different policy for outside immigration does not result in "discrimination" by ay normative perception on the interrests of nation states or (con-)federations thereof.
Perhaps, but that is not the defacto state YET.