If you are asking about software to do something you are probably way behind the position you would want to be in to start such a project.
There are three main approaches.
1) You build it all yourself from scratch. Most customisability. Nobody will use it though because just you are responsible for it and finding your OS unsupported is no fun at all. I would start with Linux from Scratch to get an idea of all the components, then you can move to include whatever you like with your own build scripts or precompiled things. Arch is the easier version of this approach as it will hold your hand more.
2) You build from a base of a more well known thing. This is the path adopted by even some of the biggest distros out there -- Ubuntu and Mint variously come from Debian, and bunch more things are opensuse or arch based. I would suggest picking the long term support versions of any of those if available.
You pick packages, create installers (Debian command line and low graphics, think DOS installer, one is great as an example) and go from there. Frankly you can do most of that from the command line in what system administrators do the world over.
3) Actual dedicated self distro making thing. The one you are most likely to encounter first is probably going to be puppy linux. https://puppylinux-woof-ce.github.io/woof-ce.html
It is mostly aimed at running from a disc/USB but people could and did add their own packages (graphics, audio, internet, office, games...) to twist it to their own ends and save those out as their own distro as it were.
People mostly do 1 if they can't find something that suits their purposes. Most commonly will be security (frankly a good install cut way down to only have the services you need will do well unless you are going up against state level actors), latency (usually for high end finance* or audio**), hard personal preference on included packages/setups styles*** (which arch and linuxfromscratch will provide equally effective routes into) and needing either specific libraries (maybe for some legacy hardware or workflow that has not made the leap) or bleeding edge versions****.
*some of the high end day traders wanting things here, though frankly most calls for things here have lost to high frequency trading and thus you would be better off getting a 4K or better monitor and having all the charts you like on that.
**average person is going to do quite OK with line in or midi in to their sound card (maybe a fancy one that happens to support Linux) and press record in audacity or whatever DAW they like. Start getting into the serious studio monitor or real time effects and you get into https://www.clicktorelease.com/code/speech-jammer/ territory which can really bother some musicians.
***init vs systemd being one of the more recent larger examples.
****while you can usually slap Linux Mint or whatever hard enough to support whatever new libraries are the hotness (justified or not) for a given application you don't want to rewrite or use an old version of it gets to be quite tedious and lots of falling debris if you go there. It is then that I usually see about upgrading and frankly it is not that hard to change distros even for your everyday driver linux (especially if you keep home on a separate partition -- though between grabbing custom fonts, browser settings and documents then most things are sorted).
My list of asterisks appears to be growing so I will cut it off there.