When I worked on a mine site in Australia, I had to design a silo to deliver a chemical into the coal preparation system. That involved making it fit into the building, ensuring that it didn’t explode when you filled it from a tanker, metering the amount coming out, stopping the chemical from getting stuck in the silo and stopping it from corroding due to the effect of the chemical. A nice easy problem for a newly graduated engineer!
The project as a whole is extremely complex, but it is broken down into small parts manageable by one person. That way many people can contribute to a really complex project. Individually the most complex thing is probably an app I was trying to make to turn a phone into a mouse for a computer. It was complex because I didn’t know how to do any of it as I started.