I'm using RedHat 9. I've downloaded lots of rpm files and
installed them as well. Reading hundreds and hundreds of
words posted on websites is causing me headache instead
of solving the problem.
Got it that:
rpm -e <package_name>
uninstalls the program.
But i think that we should know the path where is the package
installed. I've uninstalled the programs situated in /opt
directory. But what is the default location where the
packages are installed if they are not in /opt.
Is locate is only the option to search them or there is any fixed
directory?

From man rpm it looks like you need something like rpm --query --list to list the files installed by the package.

You can simply do a search to see where a particular thing is installed. There are conventions for this, and there are exceptions.