I didn't personally build my present streamer although I've customized it further from point I purchased. This particular streamer was built by one of the leading members over at audiophilestyle forum. Generally you'll find custom build streamers using some kind of atx motherboards, most of them will have upgraded internal switching power supplies powered by external linear power supplies. ATX motherboards require a number of different voltages for various components which complicates the power supply. Most builds incorporate upgraded internal switch mode power supplies powered by external linear power supplies. I upgraded this power supply via JCAT Optimo ATX full linear power supply, this over $6k so you get the idea of complexity. So power supplies keep noise down. Next you'll generally find modified Windows or Linux operating systems to disable many functions of a general purpose computer, again this to minimize processor work load and minimize noise. My build uses Euphony operating system which goes even further than the modded Windows or Linux in disabling various functions. Euphony also incorporate a Stylus proprietary music player or one can use Roon, and both can be separated for a two streamer setup, which means Cores for the first in line streamer, this now becomes a server, second in line streamer is the Endpoint for Roon or Stylus. Euphony OS also has option of incorporating on board HQPlayer if one has need for DSP engine. ATX boards also incorporate pcie expansion slots which allows for things like customized usb, I2S, network cards, these from companies like JCAT, Pink Faun. I incorporated JCAT XE network card which offers superior network filtering and clocking, this also powered via external lps vs powering off motherboard. All these various cards can be powered externally. And then we have RAM mods, these generally going to Optane industrial grade RAM, Euphony also incorporates full RAM play in which one can fully buffer music files to RAM so direct play from RAM. Generally you'll find fairly powerful processors in these builds in order to maintain a lower level usage of processor resources, this for seamless experience and lower self generated noise. I generally see less than 1% use on 7 threads in my processor, this with Roon.
And the above may have neglected some things, so as you can see complexity abounds. With these custom builds one take it further than most off the shelf streamers, My particular build was directly compared to many off the shelf, this up to Aurender W20SE and found to be superior to all, this prior to my mods. Custom builds can go further than mine via Taiko Extreme clone mods,
Previous to this streamer I went through a series of modified Mac Minis, each one going further than previous build. This simpler path than ATX builds, and these can provide excellent streaming sound quality. The problem with these is no longer much support for audiophile mods with Minis.