Remember in the movie "Apollo 13" when a guy dumped a bunch of miscellaneous stuff on a table in a room, held up two incompatible filters, and said, "We have to make this attach to this other thing. This is what we have to work with. Figure it out."
That's all there is to scratchbuilding: You know what you want (references). You know what you have (scraps, bits, pieces of miscellaneous STUFF you have been saving for (sometimes decades). And tools.
I never throw anything away without taking it apart and scavenging interesting looking bits. And you love small diameter wire, with or without insulation.
The burner can for a recent project was a brass casing from a .45 cartridge. The intake trunk was a flared case from a .22 magnum. The scratchbuilt cockpit of my 1/72 Mosquito is all bits of plastic sheet of various thickness and tiny bits of wire.
We humans have been doing this since we were prehuman: Oggh looks at a rock and a stick, and sees a really effective club/ax/mashee-niblick.
Like a really good stew, it's made from what you can scratch up…