Just got a response from the screen's manufacturer. They inform me that there are in fact 2 different USB-C to USB-C cables.
The ones I used (one came from Apple with the MacBook, they other from Anker) are apparently only for power and "low" speed data transfers .
The "right" cable allows monitors to be connected and data transfer up to 40 Gbps.
Just ordered the latter and will update this post.
Here's the complete answer received:
USB-C video input not working (MAG272CQR)
1. Please use connect with Thunderbolt 3 port on Macbook Air M1.
2.Please use USB-C Digital AV Multiport Adapter
https://www.apple.com/shop/product/...87a7a47ba68fa300f2e8cbae968a8a0904aec6c533a2f
It is specification for Macbook Air M1.
1.Native DisplayPort output over USB-C
2.VGA, HDMI, DVI, and Thunderbolt 2 output supported using adapters (sold separately)
https://www.apple.com/macbook-air/specs/
3.Please check this USB-C cable must be support display function first and refer below link as attachment first.
https://support.apple.com/en-us/HT208368