Before anyone gets the idea and goes to the trouble of building a custom ROM - install a /e/GSI for a quick sense of achievement.
The bootloader of the Xiaomi Redmi 9AT (dandelion), originally shipped with Android 10, Android 11 also already available, needs to be officially unlocked via a Xiaomi account first. The waiting periods imposed by Xiaomi on the user vary depending on the model.
Even if not absolutely necessary for flashing with GSI, get an [unofficial] TWRP recovery or TWRP fork Pitch Black recovery
It should be taken into account that the system works with the partitioning system, a so-called Super Partiton | Dynamic Partition, in which the otherwise usual /system and /vendor and /product partitions are combined.
The Redmi ‘9A’ has the architecture A64-AB (also may be called arm32-binder64-ab) starting from Android 10. For safety’s sake, check with Treble Info App whether this also applies to the Xiaomi Redmi ‘9AT’.
If something goes wrong, just reload the stock ROM with Xiaomi Flash Tool
Thank you e-Team for /e/OS GSI