How to Block installing another app in android devices?
"i don't want any app to get installed on it..."

Technically this is impossible, since there are system apps like SystemUI and framework-res that Android needs to actually function.

But if you are talking about non-essential apps only, then perhaps you can delete them with root. Go to /system on a file manager that supports root, e.g. X-plore and Root Explorer, and delete the apps that you think are safe to delete. Be careful though, as those apps are on the /system partition and not /data for a reason. Your phone might become unstable after this.

The list of safe apps varies per manufacturer, but remember, Google is your friend!

Good luck.