For me the biggest learning experience was building a system for a company, (a moderately complex CRUD app at the end of the day) and maintaining it for 4+ year

For me the biggest learning experience was building a system for a company, (a moderately complex CRUD app at the end of the day) and maintaining it for 4+ years. If I had to go back and modify the code, if I didn't understand it straight away then it was usually worth refactoring. It was my own code so I had no one else to blame if it was crap. That probably has expanded into reading others code and seeing better ways of doing things like you say.