1/ countries have market standards 2/ countries may open their market to outsiders but they must meet their standards. This requires border controls as outsiders are outside the reach of the importing country's authority.This requires a lot of experienced staff and infrastructure

inspection holding sheds etc to ensure compliance before releasing onto the market.
3/ countries that do this will also encourage or discourage their businesses buying from outsiders by applying taxes on what they bring into the country.

If a member of the WTO there'll be a limit as to how much they can discourage with a members' agreed ceiling on the tax. This to prevent the trade wars of history. Tax (tariff) can be zero if the full origins of the goods can be proven to be from the country the agreement is with

In all cases buyers must make legal declarations for every commodity brought into the (customs) territory from another (customs) territory. This too requires much experienced staff & infrastructure,sealed holding sheds etc to ensure relevant taxes paid before release onto market

4/ Countries may decide to move beyond these free trade deals by combining customs territories to encourage more trade between them. They do so by removing bureaucratic & costly legal declarations for items of commercial value goods at the borders across each other's territory

and decide on a common external tariff rate for anybody else wanting to sell on their now combined market. Goods now move between each as if *their* own goods . Free circulation around and within a customs union.
This requires *reduced* staff and infrastructure.

The more this customs union expands the less the staff and infrastructure needed accordingly.
5/ Countries may decide beyond a free trade deal & even beyond a customs union by combining their market standards territories to encourage yet more easier trade between them.

They do this by removing bureaucratic & costly compliance checks for products/produce/services/workers at the borders across each other's territories by mutually recognising an overriding authority with representatives from each country that can equally judge on buyer or seller

placing defective goods or produce on the combined (now a single) market.
Goods, service operators, workers now move between each country as if *their* goods, service operators, workers. Free movement around and within this now single regulated internal market.

This requires yet more *reduced* staff and infrastructure. The more this single market expands the less the staff and infrastructure needed accordingly.

UK has nearly 50 years being part of this trade bureacratic & infrastructure major costs reducing process.

It has now abruptly reversed to position 3 above with its extra major demands..
with the "yet more reduced (experienced) staff & infrastructure" lack of supply.

AND where part of its political territory with another has a UN brokered peace also contingent on stages 4 and 5 above