How can I improve my effectiveness in conducting sprint demos or demos in general?

Being a mobile engineer is great for demos as it's very concrete. For the most part, you are simply taking what you are already doing when recording a test plan and polishing it up. If you want to play it safe, you can use a pre-recorded video, and if you want to make things more impressive, you can try it live (make sure to go through the flow many times though before presenting to make sure things don't break during your demo).

Here's another great resource to dig into about this topic: "How to improve public speaking & your presentations?"

When do you demo your work? The context matters here since there were not too many live demos from my experience at Facebook (unless you volunteered for it).

If you're talking about async demos, which are recorded, those are much easier. Figure out a scenario that shows the power of your new feature or tool, and do as many videos/screenshots as needed to show it off appropriately. Doing this in pull requests (as Alex talks about in all his test plans) can really help both you and others understand what you're doing.

One tip I can share is to put on your user hat and talk about the work you're doing from the perspective of the end user. They don't care about the backend or code -- they care about getting something done. So embedding that narrative in your demo is very powerful. This is what the best presenters do: start off with a story and how Bobby, a new father who lives in Michigan is trying to buy diapers, etc.

Another tip for live demos is simple: practice! Grab a fellow engineer or PM and go through it with them!