I always recommend to run Google Ads straight from their self-serve platform. You can find it at ads.google.com
Doing it from Shopify is like the highly simplified “lite” version. However, since you have competition in any niche and many other businesses running ads, that way doesn’t do the trick oftentimes.
Running ads through the Google Ads platform allows for a LOT more features, control, optimization and granularity. I’m not 100% sure if you can pick specific products from within Shopify, but you certainly can on the Google Ads platform itself.