I can confidently say a few things about working at a big company:
- I have created whole products from scratch by myself with my tech lead doing code reviews
- I have worked on "legacy" code as well
I find both satisfying for different reasons. Legacy code has a bad rep, but "legacy code" should really be called "valuable code", because it has already demonstrated its market value. It has nothing to prove. I've learned things from reading legacy code: little stuff like language features, and bigger stuff like architectural tips.
Both are different, both can be rewarding. Greenfield isn't synonymous with better or more fun. I personally enjoy being the commando[1] (with infantry tendencies as well), but the other stuff can be good, too, and we each have different preferences. That said, being the "commando" can be exhausting, and I wouldn't want to do it all the time.
Would you rather work on a part of a big product or create a complete product yourself/with another engineer?