NewPipe is a popular open-source client. While the latest versions require newer Android builds, you can often find "Legacy" or older APKs on GitHub or F-Droid that still support Android 5.0.
Many Android 5 browsers are outdated, but can play YouTube via m.youtube.com with extensions. Install uBlock Origin to block ads and improve performance.
: If the app no longer opens or updates on your Android 5 device, you can often still access YouTube by visiting m.youtube.com through a mobile browser like Chrome or Samsung Internet. Google Help Features Missing on Older Versions