I haven't assembled PC in long time, by now I'd expect you can build passively cooled PC if you use PC just for some regular office job and surfing without gaming and resource heavy apps or we are still not there? Back in the days undervolting was for me for interesting than overclocking to make PC as quiet as possible.
Some passively cooled PCs exist, but they're niche and expensive.
It's a lot easier to make one that's quiet, but not silent. Big fans that spin slow and not going crazy with bleeding-edge hardware do wonders. Yeah, if you really want, you can try to underclock/undervolt some things here and there.
I think it's useful to draw a distinction between a silent computer and one that is merely inaudible. For a typical PC, all you really want is for it to not noticeably raise the noise floor of the office it occupies. Going down the route of truly silent PCs forces severe compromises, and in almost every use case it would be better to have fans, properly configured to always spin, with a minimum speed low enough to be inaudible.
