This was tough for me to get exactly right. Very little in the way of guidance via Google. I hope this helps others.
As D
I took the answers from Nathan Shephard and Dan Cornilescu and condensed them into the app.yaml below. It seems to be working for my React SPA on GAE Standard. No application server (ex: serve.js, ExpressJS, etc) is necessary.
env: standard
runtime: nodejs10
service: default
handlers:
- url: /static
static_dir: build/static
- url: /(.*\.(json|ico|js))$
static_files: build/\1
upload: build/.*\.(json|ico|js)$
- url: .*
static_files: build/index.html
upload: build/index.html