Can you help to name it in English?
The router can work in Router mode, or bridge mode. In all modes, you need to change the network typology, i.e. you need to reconnect the wires.
In “this mode”, you don’t need to touch your network setup. You only need to connect the WAN port of the GL.iNet router to your main router’s LAN, it will take over the network gateway. All of your data will go throught the GL.iNet router. VPN, DNS, Adguard etc. all works.
Yes the GL.iNet uses ARP spoofing and tell every device that it is the gateway.
Some vendors call it “simple mode”, which actually does not explain too much.