essentially, cherry mx style switches consist of the following parts:

1. housing, top and bottom;
2. slider;
3. spring; and
4. contact leaf.

hence, making a custom switch simply means you play with the above, by swapping parts from different stock switches, or after-market parts.

why are we so gatal and make custom switches? when you use a mechanical keyboard with stock switches long enough, you might realize that there are common 'issues' about them:

1. spring weight. some are too light, while others are too heavy.
2. smoothness, softness, and distance of key travels. these are related to sliders.
3. characteristics of switch constructions, e.g., different housing materials sometimes produce undesired outcomes. (nylon housings are softer, but relatively scratchy; clear/hard housings are smooth, but relatively louder, and once they are opened up for few times, switch top wobbles will be more obvious)
4. sound profile, e.g., stock springs are too pingy at times; unlubed switch housings make more noise.

even though today we have more selections compared to years ago (e.g. more variants of stock/boutique switches), there are always something missing in them. the combination of parts in a stock switch just don't work for us in one way or the other. whether expensive or not, these stock switches are just not optimal for everyone.

mkbs are long-lasting, which means you will use it for years after you have it built. so it's either you try your best to swallow and adapt to what's offered, or make necessary adjustments to make it more comfortable for long-term use. making own custom switches is about the latter, i.e. fine tuning switch feels and sound according to own preferences, so that we can have better typing experience in the long run.

for linear switches, you mainly play with housing, slider, spring, and modifications:

1. housing - my personal take is nylon housings, because usually they don't need switch films to minimize switch top wobbles. (clear or hard switch tops, on the other hand, usually need switch films)
2. slider - currently for linear switches, there are three main variants: normal, speed, and silent. usually stock sliders are fine, but you might wanna explore other variants as well.
3. spring - to me spring weight and lubing are important. my favorite range is from 60g to 67g (bottom-out weight; ymmv); i prefer heavier weight for space bar (like springs from cherry mx black; for better kick-back feel).
4. modifications - there are many mods invented by mkb community. to me, the two common mods, switch lubing (to give smoothness of key travels, and change typing feels), and adding switch films (to reduce switch top wobbles) are usually good enough.

in essence, just like mkbs, never think that certain switches are bad, until tunings are done to them.

also, making custom switches are not necessarily expensive. with the right combinations from cheap switches, outcomes of custom switches can be much better.