So where do they gain that passion if not from developing off hours?

If you look at the leveling or hiring guidelines for any major tech company, what separates “senior developers” and up from mid level developers are system design, “handling ambiguity”, leadership, “scope”, “impact” and in the case of Google did you write a messaging app.

Even the largest tech companies realize “coding ability” only gets you so far and most developers who want to get ahead would be far better off focusing on the other aspects of software engineering.

System design, handling ambiguity, scope and impact are all things that are taught extremely well by just coding a lot. I'm not even sure why you are arguing. Someone who does something more and likes doing it in general will be better at it then someone who spends less time on it and doesn't like it.

So how do you learn “system design” - infrastructure, proper data storage for the problem domain, balancing the trade offs between costs, RPO and RTO, security, caching, sharding to handle scale, scaling, disaster recovery or the stereotypical “how would you design Twitter” System Design tech interview.

Coding has little to do with handling ambiguity.

While I have never been through a BigTech coding interview, I have been through (and passed) a BigTech system design interview, it has little to do with coding.

Designing (and redesigning) systems for large organizations is my $DayJob where I have to be able to explain those concepts.