Could I use dual Ethernet connections at the same time on a Mac mini 2020 which has Apple silicon M1 inside

• You can run Wi-Fi and Ethernet concurrently for those services that need Wi-Fi such as AirDrop, etc.
• The Network preference will allow you to multi-home your Mac so that if you have two ISPs, two ISP modems, two routers, and two unique static network addresses to those respective routers — all from a single Ethernet adapter. If those two routers were fed into another router (one that can do Border Gateway Protocol (BGP)), and linked to your Ethernet adapter, then you might have network failover configuration when one ISP feed drops.

Although I have not configured a multi-home solution here, my Synology RT6600ax router has a provision for attaching a USB3 cable from an external 5G (e.g. T-Mobile) hotspot and internally configuring that for failover if the main Comcast feed drops.