Start with the cheapest thing
Try a different cable and a different plug, ideally ones you know work with another device. Cables fail at the strain point where they leave the connector, and the failure is often invisible.
If a second cable fixes it, you are done and it cost you nothing.
Then look in the port with a light
The single most common cause of a phone that will not charge is compacted lint. It builds up over months, pressed a little further in each time you plug the cable in, until the connector can no longer seat fully.
The tell is a cable that only charges at an angle, or that feels shallow going in. Shine a light in and compare it with the speaker grille next to it: if it looks solid rather than hollow, that is not the port, that is a year of pocket fluff.
We clean these out in a few minutes. It is the cheapest job we do and it fixes a surprising share of phones people were ready to replace.
How to tell a port fault from a battery fault
A worn port is intermittent. It charges sometimes, or only when the cable is held, or only on one particular charger. A failing battery is consistent: it charges fine and then loses it all quickly, or dies with charge still showing.
The awkward middle case is a phone that is dead flat and will not wake. That could be either, and it needs a bench check to see whether the board is drawing current at all.
USB-C and Lightning wear differently
Lightning puts the contacts on the outside of the plug, so they wear and they collect debris. USB-C puts them inside the socket, which protects them but makes the port harder to clean and easier to wreck by forcing a cable in at an angle.
On both, the port is soldered to the board rather than plugged into it, so replacing one is a bench job with heat and a microscope rather than a parts swap.
What we do when it comes in
We check whether the device draws current at all, which separates a board problem from a port and battery problem immediately. Then we clean the port, test with known-good cables and a bench supply, and only then quote.
The diagnostic is free, and if the answer turns out to be lint you will be told that rather than sold a port.