CleanShotX [0] is what I used. Honestly the cloud hosting aspect is overpriced but it does make sharing a picture/video super easy for cases like this. I know macOS has annotation tools built-in for screenshots but CleanShotX's tools are about 1 billion times faster/easier to use. I use this tool all the time just to take a quick screenshot, add some arrows/circles/text/numbers and send it off.
Not affiliated in any way, just a big fan of any tool that lets me quickly explain or document something. I'd say about 99% of bug tickets I enter into our ticketing tool have a CleanShotX screenshot attached (or video).
Here is an example of a handful of the tools (numbers, text, boxes, arrows, highlight area, and redact): https://cs.joshstrange.com/kSpsv7DG