The tree refers to the Linux kernel Git repo. On Linux normally all drivers are in there.

I think that is a pretty crazy concept, but it kinda gives trust and if it is in there is will likely not break.

Out of tree means the driver is not compiled in (like BTRFS on RHEL distros) or cannot even be included as it is proprietary or else (NVIDIA, Displaylink, Virtualbox).

These drivers are added locally using kmods OR akmods, I dont know the difference and never used it.

uBlue adds some drivers during the build process which is pretty cool. So even though it is out of tree, it gets centrally included and if it breaks you dont get the update.