cusotm roms dont include firmware usually.you can flash the latest china firmware yourself with twrp,thats what i did.Does the Arrow OS OTA contains the latest firmware weekly china build/global?
Currently I am on GAPPS 18/06 build came from base of latest Miui 12.5 Indian version
I hear many comments that proximity sensor works better in calls with latest firmware.
This is china weekly branch of 18/06 (https://xiaomifirmwareupdater.com/firmware/alioth/weekly/21.6.16/) whether it can be be flashed over this custom ROM with ARROW OS recovery itself or twrp needed or via adb/fastboot?
I am using Indian Model whether it will be any issue?
the indian model is aliothin though so you need the firmware for that since the modem is different i think.