[pjc50]

Well, those aren't the hard bits, you can generally pull them out of a textbook or reference design and expect them to work.

You're overlooking the entire field of analogue ICs. Again, there's a fairly good cookbook of reference circuits out there. In some cases manufacturers will help you a lot: TI have a tool for designing switchmode power supplies around their chips.

So you can compose circuits into schematics. You don't usually compose layouts as it's more compact to hand-redo them for your particular board shape. There is some open source hardware at this level; often to be found on tindie or via hackaday.

Then the other obstacles to shipping a product: regulatory approval. EMC testing. If it plugs into the mains, a whole bunch of safety testing. In the EU, CE marking and WEEE registration. If it has a radio, radio approval.

Economy of scale matters as well. That's why things like OpenWRT are so popular: repurpose hardware that's already mass-manufactured for a slightly different purpose.