You were misinformed on Brexit and border control
Being part of the EU grants relevant EU citizens freedom of movement (through border control) and freedom of employment across EU member countries
As demonstrated during covid EU member states could also close / control travel through their borders
Travellers with EU passports could typically drive down the motorways across Europe and wave their way through airports & ports, but Border Force could choose to check and also have an eye out for particular undesirables on flight and ferry manifests. The carrying airline , ferry company etc were/are held liable for ensuring passengers have suitable passports (or suitable national ID where applicable)
A particular area of concern for immigration was the overstayers of those who admitted legally and didn’t leave - nothing to do with the EU or Brexit
Today the same situation remains but EU citizens have the same status as the rest of the world, and we are part of the rest of the world when going to the EU
Undocumented migrants on boats, hidden in lorries etc have exactly the same status as they had before Brexit, and the UK has exactly the same responsibility for them …. Except for one thing, under the EU we could oblige another EU state to take responsibility for them
(Bearing in mind that nobody was required to claim asylum in ‘the first safe country)
Another thing that may have triggered the post is the recent FOIs on ‘illegal immigrants’. If they arrive by boat, and are picked up by the authorities then they have not illegally entered the UK