We have a TV we use as monitor at my office (TLC C825), and this TV is connected to our copany wi-fi, which means anyone at the office can cast random youtube videos to the TV.
Is there a way to set a password or something to block people sharing wi-fi from casting unwanted YT videos? I downloaded Google Home app but I can't seem to find an option for that...
You can disable Google Cast functionality by disabling the app named "Chromecast built-in". I think it's the best solution in your environment since users on the same subnet are trusted and can't be changed.
If you want the cast function still available, you will have to setup it's own SSID and vlan. I have all my AV equipment on it's own vlan. To cast to it, you have to join that SSID, which in effect is what you are looking for, pinned access to the chromecast.
If you do not care about the cast function, then just disable it in the AndroidTV settings under "Chromecast Built-In" section.
Never tried it myself (i should) but you can disable guest mode from the Home app.
https://smarthomestarter.com/how-to-make-chromecast-private/