Your misunderstanding is understandable.
I tried to make it look like I have the big thing by faking early building steps (I specifically bought those grey 2x2 Technic bricks just to make this picture…).
MOCs (My Own Creation) are custom brick builds by fans.
Kids are basically building MOCs all the time. (Adults on the other hand often buy sets like this Millenium Falcon for 850 EUR to build it exactly once, put it in a display case and never touch it again.)
People (kids plus adults) building more sophisticated MOCs may upload them to sites so others can copy their build. I’m not that creative so I’ve used such site for the skyline above: LEGO MOC Rome skyline by benbuildslego | Rebrickable - Build with LEGO
There you can buy and sell instructions for MOCs and also a lot of them are free.
You can search for MOCs which are based on parts you own or alternate builds for specific sets, e.g. you’ll find a lot for 3-in-1 Creator sets like this one
There are other sites selling just instructions for MOCs too, but then there are also sites selling them as sets so you don’t have to gather all the parts yourself (e.g. https://buildamoc.com/ or https://www.mochub.com/).