Building Realsense that works with the T261/5 module in the big 2025 is painful to say the least. Since T261's software support is crap in the recent versions of librealsense, I had to build an older version (v2.51.1) of librealsense that still supports T261/5 and use an older version of Ubuntu to get it working. I have tested this build on both ARM (raspberry Pi 4) and x86 (Ubuntu 20.04.6 LTS).
This document is centered around linux, if you're on windows, this probably isn't for you. But this version definetly supports the T265/1 even for windows so at least thats helpful!
On plugging the T265/1 IMU for the first time, it shows up as Intel Myriad VPU [Movidius Neural Compute Stick] but after you
run any librealsense binary it'll say something along the lines of Intel Corp. Intel(R) RealSense(TM) Tracking Camera T265. This is because unless the librealsense initilaizes it, the IMU stays in bootloader mode and won't show up as the actual camera!!.