I am new to Joomla and made stupid mistakes of editing Cassiopea template without duplicating it first. I just realized that I should have done the latter while I was digging deeper on how to edit the site through backend. My page is now a mess and I really don't know what to do next. I am trying to follow a tutorial on YT to get me acquianted to the CRM's features and nothing is happening. Is there's any way to reboot Joomla, like a really clean slate, where I could be taken back again to that part which I was receiving tutorial pop-ups and the cassiopea template is in default form.
I would suggest you take a good back up before you do this.
From your description it sounds like you were editing the core files of Cassiopea and not using the Override system and got yourself all mixed up.
You can reinstall the Joomla 4 core files quite easily by going to Joomla Update in the Admin Dashboard or under the System menu.
There is a button there that will say 'Check for Updates'. Press that button and the 'Check' will do its thing and if you are on the current version of Joomla 4 the the page will say "Currently no updates available".
However the button will now say 'Reinstall Joomla! core Files' and this will copy all the original core files and that will replace any updates to the core files that you have made.
If the 'Check' finds the next version of Joomla! then follow that process and it too will replace your core files just with the latest version of Joomla.
And for completeness in this answer, you could also use the Upload and Update button at the bottom of the above image and this process requires you to manually download the Joomla installation .zip for whatever version you are currently using, and install that file and it too will replace all the core files.