Old phones make surprisingly good security cameras. The lens is decent, the night mode is usually better than budget IP cameras, and the processing power is overkill for the job. The main limitation — a degraded battery — doesn't matter if the phone is plugged in permanently.
The app to use: Alfred Home Security Camera. Free, works on iOS and Android, and the setup is genuinely five minutes. Install Alfred on the old phone (camera) and on your current phone (viewer). Sign in with Google on both. Done. You can view the feed live, enable motion alerts, and watch recordings.
For placement: the phone needs power and a decent view of the door, driveway, or room you want to watch. A wall mount or a window suction cup mount works well. Make sure it's on Wi-Fi, not cellular — the old SIM doesn't need to be active.
If you want local recording instead of cloud, use Camo (iPhone only) or DashCam (Android) to record directly to a local NAS or SD card. Home Assistant also has a Companion app integration that can ingest the camera feed into your smart home dashboard.
One thing to check first: if the phone has a severely degraded battery, it can swell over time, especially when charging continuously. If the back of the phone is already bulging, don't do this — that's a fire risk. A phone with normal degradation (just slower and shorter battery life) is fine.