In high-level diagrams, which I think is what is being discussed here, I like to think that A --> B means that A "uses" B in some way, and leave it at that.

I do similar, but frame it in terms of dependencies.

The database can live without the web server, but the web server doesn't work without the database.

Therefore webserver ---> database.

Key thing in that these deployment / context / container diagrams don't have a temporal axis. If you want to represent a flow, then you want a diagram where time has directionality, like a sequence diagram.

yup, A interacts with B with the interaction originating from A.