set up *nix computer to only allow local WiFi network sharing and no internet access

Just remove the gateway from the lan settings.

Or iptables, but that's more effort and a new tool to learn.

What are you trying to accomplish?

Parameter that can be change to not get direct internet:

• submask to 0 (no routing)
• no default route

Tools:

• web Cockpit
• web Webmin
• Nmtui
• NetworkConfiguration

Are you sure you want to do it at the device level?

This is something that you want to do with the router, not that it's impossible...

Purchase and install the firewall-software called iptables. It allows for precise configuration; e.g. blocking any connections to the wider web. May the FOSS be with you!