Not every old phone is worth the effort. These five devices have mature Lollipop support and active communities:
Installing an OS from a zip file (often called "flashing") generally follows one of two paths: Method 1: Sideloading (OTA Updates) --- Download Android Lollipop 5.0 Os Zip File
Join our discussion on the /r/androidafterlife subreddit. If you found the right zip for your obscure tablet, drop the link (verified only) in the comments below. Not every old phone is worth the effort
Downloading a specific "Android Lollipop 5.0 OS zip file" depends entirely on your device model, as firmware is not universal. You can find official files for supported legacy devices at the Google Play Services for Nexus and Pixel page. Where to Find the Download Downloading a specific "Android Lollipop 5
Connect your device to your PC using a USB cable.
If you absolutely must have Lollipop for app testing, use the in Android Studio (create an AVD with API 21). It’s safer, faster, and reversible.