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 spent some time on this and this works perfectly for my React CRA on GAE Standard.
runtime: nodejs10
service: bbi-staging
env: standard
instance_class: F2
handlers:
- url: /(.*\.(gif|media|json|ico|eot|ttf|woff|woff2|png|jpg|css|js))$
static_files: build/\1
upload: build/(.*)
- url: /(.*)
static_files: build/index.html
upload: build/index.html