You could make in a garage some decent analog integrated circuits, e.g. audio amplifiers or operational amplifiers or even radio-frequency circuits for not too high frequency ranges.
However you cannot make useful digital circuits. For digital circuits, the best that you can do is to be content to only design them and buy an FPGA for implementing them, instead of attempting to manufacture a custom IC.
With the kind of digital circuits that you could make in a garage, the most complex thing that you could do would be something like a very big table or wall digital clock, made not with a single IC like today, but with a few dozen ICs.
Anything more complex than that would need far too many ICs.
However you cannot make useful digital circuits. For digital circuits, the best that you can do is to be content to only design them and buy an FPGA for implementing them, instead of attempting to manufacture a custom IC.
With the kind of digital circuits that you could make in a garage, the most complex thing that you could do would be something like a very big table or wall digital clock, made not with a single IC like today, but with a few dozen ICs.
Anything more complex than that would need far too many ICs.