AWS charges you for storage, so even if everything is turned off you might have an old snapshot hidden somewhere racking up a few cents a month.
I've found AWS billing to be annoyingly opaque when you're trying to figure out what a service will cost you next month. I guess people who use it a lot have a system, but as a small fry newcomer they seemed really shy about what stuff cost.
Every time I have to update something on GCP App Engine, I end up creating a billing account so I can use Cloud Build for a Node.js app, then blow the billing account away after the app is deployed. It scares me to create a way to be billed when everything I'm doing is at the free-tier level.
On AWS I discarded my account entirely after I got a bill for a few cents when I wasn't (knowingly) using anything at all.