Where did you get your information that airliners send planes back to Boeing for maintenance? My quick search tells me that they generally don't, and they either do it themselves, or rely on third parties called Maintenance, Repair and Overhaul (MRO) providers for heavier maintenance. In the case of United airlines, their MRO provider is called United Technical Operations, their own division.

https://www.vanityfair.com/news/2015/11/airplane-maintenance-disturbing-truth
https://simpleflying.com/aircraft-maintenance-checks/
https://en.wikipedia.org/wiki/Aircraft_maintenance_checks

This is not true at all. You're right that planes aren't like cars, but airlines absolutely do their own maintenance. The maintenance program is initially provided by Boeing and modified by the airline based on statistical monitoring of issues.

The entire field of reliability-centered maintenance comes right out of aircraft maintenance in the 60s and 70s, term itself was penned by people working for united. It's responsible for massive improvements in aircraft reliability, there's a reason that you can point out specific events like this in the modern era.

On a different note, a lot of the guys I worked with out of uni were all aircraft mechanics who had served in the air force.

I knew a mechanical engineer that worked for an airline doing repairs. The plane would only go back to Boeing under serious need

This is certifiably false information and seeing this sort of disinformation spread with this amount of certainty is disgusting.

Source: Aerospace engineer working for a competing Prime.