The legal definition of manufacture is creating product with the intent to sale. Courts, through the times, have set a precedence of tests to determine who is/is not a manufacturer.
The anti-gun legislators purposefully choose the word "manufacture" because they were not willing take the 2nd amendment head-on. CA guns laws are legal (not my view however) because these are rooted in public safety and intra-state commerce of which do not conflict outright with any constitutional rights.
If you finish an 80% for personal consumption, you are not legally a manufacturer. You would fail all the tests... no employees, no revenue stream, no manufacturer's license, not operating illegally because you don't have such license, no commercial presence as in no shop, no material purchase contracts, no depreciation of capital assets, and no tax filings as a manufacturer (and legally too).
Of course, our state government wanted to ban building of personal firearms. They didn't want to handicap themselves in a court fight so they did it these way. They know over time, the public and the courts would accept their definition of manufacterer... incrementalism.
It works on us, so it should work the on the public at large.