OxygenOS 12 C.60

Carriers don't touch code.  Samsung, Sony, HTC, LG, and Motorola all released timelines on how their update process works.  All of them DON'T Give source code to carriers to work on.  Their software is proprietary, and people think they would allow someone to touch their code?  That's like Microsoft giving source code to Dell, HP, Asus, and such while expecting them to keep that code secure.  More hands and more eyes in software development always leads to leaks especially when given outside the company.  This has been beaten with a dead horse since Android 4.0.  Google before that thought the same as everyone else UNTIL they did an AOSP project with Sony. During that project they found the carriers touch 0 code and only test the update against their network security.  This often took no longer than 1 month when submitted.  The longest delay was governmental bodies like the FCC testing, Wifi, Bluetooth, and such consortium.  This could take months.  To combat this since Android 5(might be 4.X) they release kernel sources after they finalized it and made Android modular so you could submit and test it before they even release the next Android version.  Every update since Android has been more and more modular allowing a lot of mainline software being able to be updated via Play Store instead of releasing major Android updates like you are asking.

The Problem always lies with the manufacturer.  Europe and Asia has the same software and bloatware yet they get updates faster…  Why?  We blame Carriers and the rest of the world realized it's OEM's.

OEM's set the rules for device support.  It's their products and all carriers follow them.  At one point OEM's would charge carriers for updates instead of updating their devices on their own.  Most OEM's only support Treble via GSI and unlocked.

OnePlus CHOSE to separate the update between T-Mobile and the rest of the world.  They could have EASILY do what all other OEM's do.  They chose to forked T-Mobile devices software and change the update cycle for it.

if someone says Android is Open Source.  You don't understand the licensing.

Kernel is GPL while the OS itself is under Apache 2.0.  You can get Android and do what you want, but features, and sofware you add DON'T have to be included if you don't want to unless they are licensed other than proprietary.  Google even sued Cyanogenmod for example for violating licensing on a lot of Google software as well as certain features you don't find in AOSP which they put into their ROMs.