How to install manjaro 20.08 headless with wifi on raspberry pi 4?

need full details on wifi configuration before boot

There is no Raspberry Pi 5 yet.

And any kind of headless configuration needs LAN connection.
So how would you set that up, before the first boot?

My guess would be some kind of NetworkManager (if using GUI setup) or SystemD NetworkD (if using CLI only setup) config.

I can’t help you with the what the config should look like, since I always use either Ethernet or a monitor to setup my devices.

On first boot, LAN is not detected - in fact light on the LAN - yellow gets blinked but not green in rpi

There is a way of doing this, and it’s called PXE boot. (Pronounced “Pixie”) but the Pi 4 needs some configuring to allow this to happen, so if you’re going to do this for 100s of Pi 4’s, it’ll be worth your while.

If it’s just 10, you’re better off following @Strit 's advice and use a monitor and a brand new Ethernet cable.