This is based on Halium 9.0, and uses the mechanism described in this page.(Now available for use in GitHub Actions)
You can download the ready-made artifacts from gitlab: take the latest archive, unpack the OTA images.zip file (make sure that all files are created inside a directory called out/, then follow the instructions in the Install section
You can apply patches yourself, just use script.
As this is based on Android 9, it is required to install stock vendor.img from Android 9. You can Android 9 stock firmware from Xiaomi Firmware Updater. GLOBAL, CHINA
Copy these images to directory out/ above together with the artifacts.
fastboot flash boot out/boot.img
fastboot flash recovery out/recovery.img
fastboot flash system out/system.img