It appears there are curved metal tabs (1 on each side of the connector) that compress until they lock the connector in place or the connector is removed, but doing so evenly as to not break off the 2 gold knife blades on the logic board that slide into the plastic connector. Both the tabs & gold knife connectors are seen in the 2nd picture, in the first picture you can see how the tabs look when they are locked into the logic board connection.

I think that apple was seeing warranty costs expand for repairs to in warranty devices where the CPU or Ram Failed and the local apple store couldn't fix it and could only give them a "new- refurbished" unit. then the broken one would have to be torn down and sent through the refurb process to sell at a discount. Huge $$ drain on in warranty repairs. So giving a modular design but not easily accessible to commoner allows the local stores to repair these issues. Also apple was seeing a large number of the old "apple" crew that was holding on to their older machines (circa 2010, 2011) and moving up to the current models because it was difficult to justify a machine that you couldn't increase ram when a newer OS version would benefit for the increase ram. Also that apple was getting the jibe of being the new Microsoft/Dell wanna be's (proprietary, not up-gradable, overpriced disposable machines.