Most of the answers are about software. Here's one about hardware, from my personal files.
In one of my early businesses we manufactured these small boards with about 50 through-hole components. Think mid-80's range.
I didn't have the money to buy any real manufacturing equipment at all. This was all self-funded, which, at that time in my life, meant thousands of dollars rather than hundreds of thousands or more.
I bought a really old Bridgeport CNC machine for $1800. I worked. The transmission sounded like it had rocks in it, but it worked. I used this machine to make a lot of our fixtures and tools. The prior owner had replaced the control system with one that took DXF files and cut one depth layer at a time. I had to design the toolpaths using AutoCAD rather than any tool that would output G-code.
I remember at least three things we made.
The first was a fixture that consisted of two rails with a groove cut into them so you could place about 25 board into the fixture and stuff the components. No budget for metal, so it was made out of pine and CNC machined. This allowed for the component leads to go through and not touch the table.
The second was a set of component benders, also made out of wood, so you could bend the various component leads as required with reasonable repeatability. The jig to bend MOSFET leads to 90 degrees was this contraption with a hinge, a bunch of springs from Home Depot and a pocket for the transistor to fit into. You'd push down on this lever and it would bend the leads to 90 degrees. I wish I had pictures. We bent thousands of MOSFETs that way.
Soldering. Boy, was this a horrific thing. Again, no money for a real soldering setup. What we did instead is build a preheating box with some stuff I got from a surplus store. We would then dip the boards (just the bottom) into flux. After that we'd dip them (again, just the bottom) into a molten solder bath. The pot was about 4 x 4 inches, just enough to have enough solder and maintain temperature. From there they went back onto the wooden frames for slow cooling. Talk about janky!
Finally, the through-hole leads had to be cut. This was not something you wanted to do by hand. We did, for a batch or two, and then decided that was enough.
We made a jig (again, wood, this time I believe it was oak) to hold about 50 boards upside-down on the CNC mill. We then got a small circular thin blade (not an end-mill, that does not work at all as we discovered) and programmed a slow lead cutoff cycle. Think of it like a cutoff wheel from a Dremel tool but without the screw sticking out of the bottom. We could cut the leads to within one or two mm from the board with reasonable consistency.
There's more. Everything in that garage was janky as can be. We eventually made enough money to move out of the garage, buy some SMT equipment, redo the designs in SMT and hire a few more engineering students to help. Crazy as can be. Had as can be. Fun too.