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