I'm not personally aware of any well-designed[1] codebases made by more than a handful of people. As far as I can tell, most good "architectural and engineering decisions" are either made by a relatively small number of people or emerge from a combination of necessity and attentive devs, regardless of whether they communicate in-the-flesh on a regular basis.
I'd be interested in seeing/reading about cases of good "greenfield" projects where major architectural decisions weren't made almost exclusively by a relatively small number of people, particularly if the number consisted of more than one digit.
[1] subjectivity aside