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-truthhttps://simpleflying.com/aircraft-maintenance-checks/https://en.wikipedia.org/wiki/Aircraft_maintenance_checks

Yeah, probably not. When you sign up and agreed to their ToS, they don't "own" your content, but you grant them a worldwide, royalty-free, perpetual, irrevocable, non-exclusive, transferable, and sublicensable license to use it without compensation.
From their ToS:
Source: A pretty good post on r/HFY, though it is on Reddit, so don't click it if you don't want to :P