Photos uses an internal Google framework, Wiz. It’s been publicly described (by one of the creators) as almost identical to Stimulus, but I’m unfamiliar with Stimulus so can only trust that it’s so. Wiz handles the controllers/models/views, and Photos leverages Closure for the templates, and JsAction for the event delegation (both Closure and JsAction have been open sourced — Wiz has not).

Google uses a variety of frameworks across our different internal and external products. Both Angular and Polymer are definitely used extensively (for example YouTube is now Polymer), and the team I currently lead actually uses React and Redux.

There is often no one right choice for frameworks, and different products have picked based on their unique needs, as well as being influenced by legacy choices or necessary integrations.