" I can tell you from first hand experience that you can sell the most mundane development work, like basic jQuery and CSS site updates, for hundreds of thousands of dollars with the right opportunity."

Where could I learn more about these kinds of opportunities? Does it come from networking, domain experience?

Definitely networking is the most important. In my example, I got referred to my first enterprise client from an old boss of mine who appreciated my work. I literally woke up to an email one day that, within a week or so, led to a contract worth about 500k over the following 6 months, and a few more clients from there. I did nothing but follow up to their intro and answer questions I was asked. I have yet to have the same experience again, because my network is not large. It was mostly right place right time. Eventually it fizzled out because I was not very into doing consulting work and preferred to focus on product-based businesses instead, spending a lot of time building new things instead of growing or improving our DX, client relationships, and lead funnels. I let things slip, managed poorly, and as a result we lost our clients.

It also comes to good sales as well. Again in my example, I was originally approached for a single contributor personal contract. It was just me at the time. Within a couple weeks of working together, I quickly realized they were bringing on these individual devs left and right. I simultaneously pitched the client on expanding our contract to multiple contributors, AKA becoming a true vendor, while also recruiting and hiring a team behind the scenes (again, networking - I pulled from past coworkers and friends who could get the job done, so the hiring process was far from exhaustive, but saved critical time). It was a risk, but it worked out and I was able to stitch together the team and win the contract. We became their de-facto “engineering team” of about 5 devs, and I set up a corp entity and began processing invoices and running payroll for my new employees.

Use accounting software from day one. Hire a corporate attorney. Worth every penny. Spend as little as you can to make your employees as happy as possible. Do not run payroll in multiple states unless you absolutely have to, or be prepared to deal with L&I and revenue departments for every state you have employees in (see: accounting software, corp attorney). Just a few of the lessons I learned along the way.