We seem to get a lot of questions from people who want to do something related to charging a mobile phone or tablet. Sometimes they want to build a charger, sometimes they merely want to buy one.
What's often not obvious to those who first confront the problem is that there's a variety of non-standard signaling between devices and their chargers which must be present, or the device will fall back to drawing around 2.5 watts in compliance with legacy USB specs. Particularly because there exist aftermarket chargers which seem to work across a variety of device brands, many site visitors tend to assume device<>charger signalling is more standardized than it really is.
In terms of various question types:
Asking what one can buy in terms of a charger or charge-negotiation module is generally off topic as a "shopping question"
Asking what a particular consumer device expects in terms of signalling from the charger seems like a question about the behavior of that consumer device, and thus an off topic "usage" question. Just as it would be if someone asked say, what their laptop expected from its charger. In some cases more device-usage sites such as Android SE and AskDifferent (Apple products) might handle these.
If the asker could clearly state the specific technical expectation a device had of its charger (eg, specific bias resistors or voltages), and show some meaningful effort towards implementing that, then there are potential challenges of that which could be on-topic design questions if well documented and supported by specific observations such as measurements of data line voltages and current drawn.
With regard to standard schemes such as USB power delivery or some of the attempts to standardized data line bias signalling:
Questions seeking general explanation of a technology aren't really specific in the sense of the Stack Exchange mission - rather they fall to traditional references first. Eg, "How does USB power delivery work?" is off topic, but a very specific confusion following after reading cited references could be on topic.
"How to build a USB-PD charger?" would seem to similarly be too broad
But a specific well characterized issue in a particular design of a charger could be on topic, if it met all the standards expected of other design questions
So how do we handle these?
Do we close them as off topic?
Do we let them slip through, only inviting more (that some have slipped through then gets pointed to as justification when more are asked)
Do we create a single canonical question that, while technically addressing something off-topic, serves to divert almost all of this traffic to a single place? That's worked out to a degree for the common question about power supply amperage ratings exceeding the need of a load: Choosing power supply, how to get the voltage and current ratings?
To be clear, this is a question for the community as as whole. It's our site, and up to us to figure out out what sorts of questions can be handled here.