Steam is not listed in the flatpak, so I'm assuming it installed the .deb version?
Try;
sudo apt install steam
steamMaybe, did you see the source on this screenshot? It is "Zorin OS", is it deb or something else?
Okay, it's just hard to see on the picture due to it was small.
So it's a .deb file 
Let's start it via terminal and see what it spits out;
steamSo if I understand: Zorin source = .deb ?
correct. It draws from a repositories full of .debs
hmm.... the crash is as you suspected related to the driver switch which somehow messed things up (The last 3 output lines tell me that).
You have now 2 options. 1) We could try the flatpak which draws its own drivers or 2) fixing the driver related issue.
If you says that the db Steam use less ressources to run, I prefer to keep it and solve the driver issue...
I checked another application I installed: Kodi: it is from flatpak source 
Do you think I should uninstall it and install to from Zorin (deb)? But it is another story...
If you want the latest version you should keep flatpak, but if you want it using less resources and also save space use the .deb
The problem from my side is I'm not using Zorin (I'm using Fedora), so it will be from my memory without I can't double check.
Lets try with;
sudo apt install inxi
inxi -GOk, but I guess I would have to configure everything again.
In the future, is it therefore recommended for a low resource PC like mine to always choose the Zorin source rather than Flatpak when it is available?
Lets try with;
sudo apt install inxi
inxi -G
Ok, it is to try to resolve the driver issue?
First line done, I type the second...
Personal opinion? Yes. Some people swear to flatpak or snap because you get the latest software. The downside is they uses more resources and takes up a huge amount of space on the harddisk.
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Personal opinion? Yes. Some people swear to flatpak or snap because you get the latest software. The downside is they uses more resources and takes up a huge amount of space on the harddisk.
Ok thanks
Done:
It will give us info about your graphic card and driver.
It says, it doesn't use any driver o_O... strange. And you reverted back to Open Source driver?
Try
sudo apt --purge remove '*nvidia*${DRIVER_BRANCH}*'
sudo apt autoremove
I don't know, I juste did what I said above...
Okay. Next is to make sure the firmware is there:
sudo apt-get install nouveau-firmware
sudo dpkg-reconfigure xserver-xorg
requires reboot afterwards.
Rebooting...
Rebooted.
Try start steam up again.