Wappalyzer works great for finding what javascript or css frameworks a company uses (i used to depend on it a lot when i owned a web agency) but it doesnt do a good a job at detecting the infrastructure a company uses. Like if they use Grafana or Snowflake or ChatGPT APIs.
Has anyone here built such a tool before? Again, something preferably open source :)
If you're looking for something truly free/open-source, you probably have to stitch together various sources. You can analyze job postings to see what technologies they mention on job ads (although this can give u some false postives too as companies love to spam long lists of technologies). You could look at what appears when they search for their domain in subdomain tools, which can often give you a clue if they host stuff like CI/CD internally. And just good old-fashioned snooping around the Linkedin profiles of engineers who work there.