My steps, if anyone has to replicate. I'm not sure if everyone will start from here... it's possible that you already have the prerequisite installed from another game. But I'll give my fix path in case anyone starts from the same place I did.
For me, what I saw to start: When running, a brief spin, and then about ten seconds later, a change back to the green play button. Complete no-go.
My fix path went:
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Started in Desktop mode, for maximum troubleshooting flexibility.
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Going to Manage > Properties, I changed BG III compatibility option to Proton Experimental.
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Tried relaunching game... this time, an error returned that I needed to run the game along with Windows Net Desktop runtime (.net 6.0.20).
This error had its own link button, but in case you don't get the link or error, but still want to see if this is your fix, the link took me here for the download.
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I added that downloaded file as a non-steam game in Steam, set its compatibility to Proton Experimental, then I ran it and installed it. (Note: For stuff like this, leave it in your Steam library after you're done with these steps... if you delete it later, it takes out the file path and it will stop working, and you'll have to do all this all over again.)
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Every person is going to get a unique directory number created for them in their file structure for this file. Find it by following the path in your file manager (default file manager is Dolphin): /home/deck/.local/share/Steam/steamapps/compatdata/
Then sort by "last modified," and the file folder you're looking for will pop to the top (it should say something like "last modified two minutes ago," "last modified just now," or something like that... because you just did this, so you know this is the correct one). Write this number down, it's the unique-to-you location id I was talking about earlier.
Finally, go back to Baldur's Gate III and the Manage > Properties. In the launch options, paste exactly this command, except substitute your unique ID number in place of the string of Xs here:
STEAM_COMPAT_DATA_PATH=/home/deck/.local/share/Steam/steamapps/compatdata/XXXXXXX %command%
(Note: there is a space between your ID number and %command%.)
And if your issue was for missing the Net prerequisite, then this should get it running for you. Have fun!
How did everyone die? Assuming that you were the rich person or the indentured servant of a rich person, it would depend on that for sure.
Was it a horrendous, highly contagious mega-pandemic that no one is immune to, and you survived because you billionaire-bunkered the moment that news reports started to hit? I'd think you could resurface sooner rather than later, and there will be places you can travel to that aren't really contaminated by the dead (like places that had low population before the outbreak).
After a few years, you could branch out to wherever (not that any single place is really that much better than others in a nearly empty world), likely the plague will no longer be virulent among the dead. You could quickly carve out a decent life for yourself, though you'd better get self-sufficient fast, without the support structures of the old world being there to do everything for you.
But if it was nuclear apocalypse? You're going to be bunkered for a long time, with little company. You'd likely end up envying the dead.