The locked-down-PC problem
The machine in front of you at work probably won't let you install RealVNC, and Apple's Screen Sharing app doesn't exist for Windows anyway. But it has a browser — and that's the whole requirement. Open the viewer, enter your pairing code, sign in with your Mac's username and password, and you're at your desktop.
Through the corporate firewall
Both your Mac and the browser connect outward over HTTPS/WSS to the relay, the same way any web page loads. There's no inbound port, no VPN client, nothing for a firewall to object to.
Full keyboard, real shortcuts
The viewer maps a Windows keyboard onto a Mac sensibly, and the toolbar's modifier keys latch ⌘ ⌥ ⌃ ⇧ for shortcuts your PC keyboard can't express. Copy on the Mac, paste on the Mac — your session behaves like sitting at it.
When you're done
Close the tab. Nothing is installed on the PC, nothing is cached, and your credentials were never visible to the relay — here's why.
Also useful: From an iPhone · From a Chromebook · Over Tailscale · How it all works