How is software architecture decided in a scrum/agile project environment?
It's never "decided". You add features and refactor and eventually the team hits on a design that stops needing to change so much to add new features.
Agile doesn't mean you don't do a design. You do all the waterfall phases. You just do them in one sprint instead of six months to a year.
It means you get something in front of a product owner that isn't just a piece of paper.
Agile puts even more pressure on the design than waterfall does. Agile needs a flexible design because there is no guarantee that the requirements will be the same from sprint to sprint.
"Who decides" is whoever turns in code that follows a design that doesn't get refactored into another design.
I've checked up on projects years after I left them. I'm not nearly as proud of the ones where I happened to get to set the design as the ones where my design lasted more than 6 months of active development.