¶ The Problem
I connected a Redmi 9(with USB Debugging and File Transfer enabled) to my Arch Linux computer, however, it was not being detected by lsblk, mtp (through the pcmanfm integration), etc. However, it was being detected by dmesg and lsusb.
¶ The Solution
After some fiddling, it turned out that the problem (apparently) was that my computer hadn’t been authorised by the phone.
I ran adb shell, causing an authorization popup in the phone. After authorizing the computer, it was able to detect the phone through lsblk and mount it through mtp=/=pcmanfm etc.