Not a good comparison, sewing machines and computers. Computer components tend to follow "standards", i.e. power supplies, motherboards, drives, memory etc can generally be mixed and matched and will work together. Sewing machines not so much. Other than some very basic components bits from one machine will rarely fit another, and while it might not be impossible to build a machine from various bits and pieces the work required would be enormous (and possibly expensive). I have some mechanical skills but building a sewing machine that way is not something I would ever contemplate! (And yes, I've built quite a few computers over the years).
You can, to some degree, change a machine to use a different class of needle (usually by adjusting the needle bar to change the clearance) but there's usually a trade-off, it might affect clearance under the feet or needle lift or the thickness you can sew. This is usually done if the machine uses some obscure or defunct needle type and is changed to a more common type.
Likewise with thread size, there is a physical limitation on the size thread a machine can use. It is often possible to go up a size (#69 to#138, for example) but it depends on the feed dog/bobbin/hook clearances, some machines can do it but some just don't like it.
With one notable exception we're generally stuck with a particular class of machine for a specific purpose. That exception is the modified needle plate/feed dog that is now available for the big 441-class machines, it makes them far more versatile than they were originally intended for - but they still won't do everything!