When it doesn't work

Six problems cover nearly everything. Start with the one that matches, and if none do, jump to diagnostics at the bottom.

The phone or tablet can't find the PC

Same network, really. The device and the PC must be on the same Wi-Fi. A phone on mobile data, or on the 5 GHz network while the PC is wired to another subnet, will not connect.
Guest and hotel Wi-Fi block this. Those networks isolate devices from each other on purpose (the router calls it AP or client isolation), so the announcement never arrives. Use your main home network, or skip Wi-Fi entirely with a USB cable.
VPN on the PC. A VPN can swallow local traffic. Pause it and retry.
The firewall prompt. If Windows once asked about CatCamHost and you clicked Cancel, allow it again: Windows Security > Firewall & network protection > Allow an app through firewall > CatCamHost on Private networks.

No CatCam camera in Teams, Zoom, Meet

Windows 11 22H2 or newer is required; the virtual camera API does not exist before that.
Restart the calling app. Apps list cameras at launch, so a Teams that was open during install has an old list.
Check the tray. The cat icon should be in the system tray; if not, start CatCam from the Start menu.

The camera is listed but black or frozen

On the device, the big button should say it is streaming; tap it once if not.
Tray icon > restart CatCam, then reselect the camera in the app.

No CatCam microphone

Reboot once after the first install. The audio driver finishes registering then; this is one-time.
In your calling app, pick CatCam Microphone as the microphone. Windows lists it as "CatCam Microphone (VB-Audio Virtual Cable)": the cable is theirs, the audio is your device's mic. If you already had VB-Cable before CatCam (OBS, Voicemeeter), CatCam leaves its names alone: pick CABLE Output instead, or run CatCamAudio setup --force from the install folder to get the CatCam names anyway.

USB cable mode

The cable path needs USB debugging on the device: Settings > About > tap Build number 7 times, then Settings > Developer options > USB debugging.
The installer's USB checkbox fetches the Google platform-tools it uses; if you skipped it, run the installer again with the box ticked.
First connection: the device shows an "Allow USB debugging?" prompt, accept it.

Still stuck: diagnostics

Start menu > CatCam Diagnostics writes a zip to your Desktop with CatCam's logs and the network basics (local IPs, PC name; nothing is sent anywhere). Attach it to an issue at github.com/igorfyago/CatCam/issues and describe what you clicked. Issues get read.