As far as I can tell, DaVinci Resolve is not available in a Debian/Ubuntu package. The standard installer, designed for Red Hat, doesn’t seem to interact with the package manager either. This makes me think some kind of wrapper script you downloaded from the internet was the culprit here.
There are some guides online that will make Resolve into a package, but they seem to be pulling all kinds of weird tricks. I would not recommend using those guides without some kind of backup and recovery tool set up for your computer.
It’s hard to tell what exactly got removed, so I don’t know what you need to reinstall. If you use a tool like Timeshift or Snapper, now would be the time to restore a previous system snapshot. If you don’t, you’ll need to do the recovery manually. Either way, this isn’t an easy fix, especially if this was caused by a script like MakeResolveDeb which seems to also modify other system files.
To get a running Kubuntu install back, you basically have two options: either use the command line to sudo apt install every package you notice missing (sudo apt install dolphin konsole…) to reinstall them, or, what I would do in your case, do a clean reinstall to get everything back in working order. First make a copy of your entire home folder (and any other folder you may want to save) to another drive, then do a clean install, and copy the files back to where they’re supposed to be.
If you can’t log in, try logging into the console (ctrl+alt+f3, type username and password when prompted). From there, you can run a command like sudo apt install kubuntu-desktop. That should fetch most Kubuntu files it it installs successfully. If it refuses because of package conflicts, you’ll need to remove the conflicting packages first (i.e. sudo apt remove davinci-resolve if apt complains about kubuntu-desktop conflicting with Resolve).
A reinstall is probably quicker and easier, but you’ll need to make sure to copy over everything (including hidden files!) you may need off the broken system. You can do this from the Kubuntu installer by running the “try kubuntu” option when prompted and simply launching a file manager. Any system modifications you made to your system (additional drivers and programs, configuration) will need to be made again. If you haven’t messed with the system too much, this shouldn’t take long; all you need is to install your old programs, and the config files from your backup should leave you right where you left off.
As for system snapshot tools:
If you’re comfortable with messing around with partition layouts, I highly recommend looking into setting up BTRFS+TimeShift; it could undo the damage in seconds after rebooting.
Unfortunately, Kubuntu doesn’t offer this tool as a simple option in the installer, so there’s a bit of manual work involves to get it to work, and if you don’t know what BTRFS is you may not want to deal with that nerd shit.
I think setting the partition type to btrfs during setup is all you need to do (that, and installing timeshift of course), but I haven’t verified that this still works.