If those two networks were still around and never merged to become The CW back in 2006. It of course, goes without saying to right away assume that certain shows that have aired on The CW would've made their home on UPN or WB first. But let's just say, they would've plausibly aired on those two networks within the 15+ timespan of them officially shutting down.

To give you a better idea, where is a review (https://www.cbr.com/astrid-lilly-save-the-world-tv-review/) of the new SyFy show Astrid & Lilly Save the World:
"All of Astrid & Lilly Save the World's supernatural elements seem designed primarily to keep the budget low, but the limitations just force creators Noelle Stehman and Betsy Van Stone to be more inventive, focusing on humor and character development rather than big monster battles," says Josh Bell. "Brutus specifically references Buffy the Vampire Slayer, referring to himself as Astrid and Lilly's 'Giles,' and Astrid & Lilly Save the World channels some of the deft combinations of teenage angst and supernatural threats of Buffy's early seasons. The writing here isn't as clever, and the mythology isn't as complex, but Astrid & Lilly Save the World works best as a junior version of the kind of supernatural dramas that The WB (and then The CW) aired for decades."

In the case of UPN, would for instance, Star Trek: Discovery and/or Star Trek: Picard aired on that network instead of the Paramount+ streaming service (formally CBS All Access)?

I think that "The Neighborhood" could have aired on UPN if it (UPN) was still around.