I really doubt that's what happened to you

Do you have screenshots?
Sorry, but no, I didn't take screen shots. I'll do it next time I try though. I'm fully aware of how hard this may be to believe without evidence, so I'm committed to getting that evidence next time it happens. I'm honestly lost as to how this stuff happened myself, it makes zero sense at all.
Also, I don't care about your doubt, I was there & saw what happened, you didn't. All I can say for certain at this time is that some of the mods I installed for Civ6 showed up in Civ5, probably caused my old game to crash somehow, so that I felt it necessary to start a new game, because trying to load it kept crashing & some of those Civ6 mods were active in the new Civ5 game so that I had to start another new game, after disabling those mods I'd installed in Civ6 via the Civ5 interface. Now Civ5, modded with Vox Populi, is working correctly again, btw.
Specifically, the three Civ6 mods I recall that showed up in Civ5, were the one that gives your initial settler a greater vision range*, Perfect World for Civ6**, and that "more advanced setup screen" mod that puts mods in 2 columns***.
Also, I checked to see where my Civ6 mods were being installed (under Steam & under Documents) and it's in neither place. I installed dozens of mods & I can't find them installed anywhere. If that helps anyone.
* I know this exists for Civ5 also, however, I didn't like it when I tried it in Civ5, so had uninstalled it there years ago. I wanted it in Civ6, because civilization placement, with Perfect World, is so much more terrible in Civ6 than Civ5, so I felt that was necessary to plant the first city where I really wanted it to be.
** When I'd started a new game in Civ5, after installing those dozens of Civ6 mods, I initially didn't understand why there was a second copy of Perfect World in my mod list, but I checked it on anyway, it was only later, after seeing that wide-vision initial settler, that I'd realized the "less good" version of Perfect World maps had been generated in Civ5. There were a ridiculous amount of rivers, which is characteristic of Civ6 Perfect World, not the Civ5 (updated) version.
***This is what drove it home to me that Civ6 mods were happening in Civ5. I've never seen this in Civ5 before, but when I started new games, that mod was happening in Civ5. I saw installed mods show up in 2 columns, which was absolutely impossible, I thought, yet there it was. As far as I know, Civ5 was coded in Lua for many of the mods (especially UI), but Civ6 switched to a different language, I don't recall what, so how the hell did that happen? I was so confused at that point. I totally understand people not wanting to believe this without screenshots, which I wish I had taken, because I also don't want to believe this. Yet, I was there for the thing and the thing happened & I have no idea whatsoever how this might have happened.