What is an example of something simple vs complex? I believe my services' APIs count as simple. HTTP verbs, url and query parameters, some auth headers and jwt

What is an example of something simple vs complex? I believe my services' APIs count as simple. HTTP verbs, url and query parameters, some auth headers and jwts, json payloads, and http error codes usually with a json body. As such, all my problems are easily solved with the chi package. I don't do much form handling as our UI handles that and uses our APIs. FWIW, our APIs handle billions of requests per day. What is complex? I imagine things around templating, forms, and maybe (maybe!!) auth. Is that what drives framework advocates? My past experience with a few were that they made simple things simple and hard things harder or impossible. I abandoned them about a decade ago however, so could be out of touch.