Hi,
API calls triggered by the backend DO count towards API usage -- so using the API Explorer to make calls do get counted as API calls. So - if you use API explorer to trigger a user batch job that then runs scripts on all of your users - yes, yes that does get counted as API calls. 🙂
Also - uploads of files to and from the portal do get counted in the file transfer costs.
Other normal portal operations - like viewing users, viewing logs, editing scripts, etc - do NOT get charged in API counts. So to clarify - using the API explorer to view a leaderboard is going to add an API count - but using the leaderboard viewer at App > Global > Leaderboards > Leaderboards does not!
To be honest - in general - usaging of the portal is negligible. You won't really notice the usage added by what you're doing in the API explorer or working through the portal. It's only if you use the API Explorer to kick off a large job that API counts would be noticable <-- as you'd expect!
I hope that helps!
Paul.