Indeed they can resource it how they feel best but the point not mentioned here is that every country in the Schengen Zone has to apply the same checks and rules so Portugal/Spain/Greece cannot create an "express" entry waiver system for UK citizens to benefit their tourism - they can open up lanes etc etc but they can't have their own rules. This is because e.g. Spain is carrying out the border check on behalf of every country in the zone.
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Point being - it depends on UK/EU future relations but if we are completely outside the bloc then it's daft to think that it won't cause border delays because the immigration rules will need to be enforced. This can be done electronically but without freedom of movement UK ctizens travelling to the EU have no automatic right to enter. Entering the EU will be more like entering the US - let's say an immigration process rather than a passport check. If the UK doesn't allow unlimited entry to EU citizens then of course no one should expect the same in Schengen and other EU countries for UK citzens.
Personally I have family residing in Spain and Germany so I'd see the above scenario as very much damaging to my own personal freedoms and I hope a comprehensive agreement is reached on this but expect Johnson et al to have to change their stance on freedom of movement for this to be achieveable.
It has already been announced that (subject to reciprocity) the EU will allow UK citizens to travel to the EU without a visa (from 2021) provided they apply for a waiver under the ETIAS scheme.
Carriers will be obliged to return any travellers who arrive without prior ETIAS clearance, so we can be sure carriers will require ETIAS details at (or prior to) check in, just as they already require passport or ID card details. No visa and no ETIAS means you won't even get on the plane.
Passenger lists will be forwarded to the destination airport (just as they are already) so the authorities will have prior notification and can check in advance to confirm all passengers have valid entry documents. On arrival passengers pass through border control who would check passports to confirm the passenger is the passport holder (just as they do now).
If ETIAS is implemented there is no reason why border control checks will take any longer than they do currently.
The EU only want reciprocity in terms of visa-free entry for EU citizens. They are not asking for Freedom of Movement.