I have bought a TP-Link Archer AX73 to improve the range and speed of the Wi-Fi in my home.
How do I put my BT Smart Hub ('Home Hub 6') in bridge mode?
If it's not possible, is it still possible to use the Archer AX73 with it?
You can't, BT hubs don't have a bridge mode.
You can, but not sure it will improve the range of your wifi.
You can either use it as an expensive Wireless Access Point (WAP) by connecting it to the hub LAN to LAN or as a router with its own subnet by connecting Hub LAN to TP Link WAN. You will be double natting by doing this.
Or buy a separate modem. Plenty of cheap second hand Openreach ones.
Why do you want two routers? Why not replace the BT router with the one you have just bought?
You may not have noticed, but that device is just a router, not a modem/ router