People were really tuning in to see how Kamala would handle herself. Trump is a known element—he's been in this sphere for nearly 10 years at this point. Everyone who is undecided was already expecting Trump to be Trump. That is, they expected someone who shoots from the hip and doesn't prepare for these things much. I think the worst moment of this was when Trump couldn't articulate what he would do to replace Obamacare—a particularly painful moment since he also got grilled over this same thing by Republicans in the 2016 primaries. But on pure instinct he's able to break framing, and from this were a few clippable moments from the night. And good rhetoric about Kamala's inefficacy thus far as a figurehead of the current regime...she says she's not Biden, but she was the VP...
The decision Kamala made to provoke Trump over the crowd sizes (alongside some other aggressive gestures) was obviously a calculated move to try and make Trump look unhinged. And I think that was a mixed bag, because Trump is actually in his best form in an adversarial setting. Some are coming away from that thinking he went too far, others are praising that performance.
As far as the actual "substance" of the debate goes, there are a lot of complexities involved in the issues that were brought up. And that renders them unable to be properly scrutinized in that debate format. The migrants eating cats and dogs thing is a culmination of unrelated (yet conflated) news stories which are circulating in social media. And Trump was incorrectly "fact checked" about late term abortions if you consider what VA governor Ralph Northam has said. Kamala's insistence that Trump was fighting border security by being against what she is calling a bi-partisan bill...which is specifically The Emergency National Security Supplemental Appropriations Act from earlier this year...was DoA for a number of reasons that have very little to do with Trump. And Trump claiming Kamala met with Putin was an obvious blunder. There's a lot more I could bring up, but this suffices to show that these are nuanced issues which could not get proper treatment.
Both sides played dirty...so people coming away from this feeling one side was lying/obfuscating—are really just not paying close attention, or not doing their own research. But that is probably why most people who are undecided...are undecided. And these debates are really more like perception checks than a forum for ideas. So who "won" in that sense? I don't think it's very clear yet which way the undecided voters have been swayed. I think both sides were hoping for a knockout moment in this, but there just wasn't any. Kamala's camp is wanting a second debate; we'll see what happens next. I think Trump will agree, he's just dragging this out to make the other side look desperate.