Is hard reset harmful for iPhone?

By a hard reset, do you mean a forced restart:

• Press and quickly release VOLUME UP
• Press and quickly release VOLUME DOWN
• Hold the SIDE button until an Apple logo appears (15-30 seconds), ignoring "slide to power off" if it appears

That is the equivalent on a PC or Mac as powering it off with the power switch, rather than the Shutdown procedure. It terminates all running apps without allowing them to “clean up” any process in progress, so it can cause data loss in apps that are running (just like it can on a computer). But it won’t cause any damage to hardware or iOS.

The better way to restart (unless the phone is frozen) is to shut down, then start. This gives apps a chance to clean up.

what exactly happens when i hard reset

Please read the Apple Recommended post near the top of this page. If that doesn’t explain it to your satisfaction post back and we can discuss it further.

A factory reset is not harmful for an iPhone and sometimes will resolve problems. It is fine to reset a phone if something isn’t working or you want to give it to someone else.