If your hotspot has a limit on the number WiFi clients that can connect to it (typically anywhere between 5-10), bridge mode will not be ideal. If that's all you need, go ahead and explore bridge mode solutions already mentioned earlier.
If you need more flexibility, install a router software on the old PC and use it's WiFi as WAN.
I've done it using Untangle NG and I would think that the same can also be achieved using pfSense or OPNsense (choose your poison). Free versions will suffice.
Three important things you must do:
- Set the WiFi interface to Client mode (as opposed to AP mode)
- Assign it as a WAN interface
- Type in the SSID, encryption type, and pre-shared key for the hotspot
- Assign one of the wired Ethernet ports on the PC as the LAN and connect a standard Ethernet switch so you can share the connection
This is not bride mode so any number of devices connected to the PC/Firewall LAN interface will be able to share the hotspot connection, with only the bandwidth limiting performance.