Phone restart again and again

Have you noticed the phone getting hot before it restarts? The Redmi 9A runs a Helio G25 chip and while it is not a powerhouse, if it is overheating it will reboot to protect itself. If warmth is a factor, that narrows things down quickly.

If heat is not the issue, think about whether you installed anything recently. Go to Settings, Apps, and sort by install date. A dodgy app is one of the most common causes of random reboots and is easy to miss. Uninstall anything suspicious and see if it settles down.

Worth trying safe mode too. Hold the power button, long press "Power off" and it will offer safe mode. Use it normally for a bit. If the reboots stop, you have confirmed it is a software or app problem rather than hardware. Then it is just a case of uninstalling recent apps one at a time until you find the offender.

Also check your storage under Settings, Storage. If you are above 90% full, Android gets very unstable and reboots are a common symptom. Free up some space and see if that makes a difference.

If none of that changes anything, try wiping the cache partition. Power off, hold Volume Up and Power together to get into recovery, and select Wipe Cache Partition. It does not touch your personal data.

Still happening after all that? Do a factory reset. Back up your photos and contacts first. This fixes the vast majority of reboot issues that are not hardware related.

If it is still rebooting after a clean reset, the battery is the most likely culprit at this point. The 9A is a 2020 phone and a degraded or swollen battery will cause exactly this behaviour. That is the point where it needs opening up and a replacement battery fitting.