Best way to let app developers know of accessibility issues in apps

When I reach out to a developer for the first time, I explain very briefly that I rely on VoiceOver to use my phone and what it does, then go into why their app isn't working for me - this is where I explain about things like button labels not being read properly and other accessibility related issues. I also mention how they can test their app with VO, though you don't have to be very detailed as Apple has a great site you can just paste the link to which explains how to use VoiceOver to test an app and how to then fix any common accessibility bugs from a developer point of view.
https://developer.apple.com/documentation/uikit/accessibility/supporting_voiceover_in_your_app

Generally, I've had much more luck with smaller companies. A small indie team of a few people or even a single developer is far more likely to look into your problem than a large company/corporation that has no previous accessibility track record, but this doesn't mean you shouldn't try anyway, and don't get discouraged if someone doesn't respond or gives you a generic "we have taken your feedback into consideration" message or worse says accessibility isn't on their roadmap.

One final thing worth mentioning is games written using what's called a game engine, like Unity. If you launch an app, and find VoiceOver reads absolutely nothing and either touching the screen just plays the empty space sound or just gets past through to the app so tapping random spots might cause something to happen, that app is written with an engine. They're great for devs because they make making an app for multiple platforms very easy, but this also means the way the information gets displayed bypasses iOS's standard user interface functions, which is why fixing accessibility for these kinds of apps is very hard for the most part. However, someone has recently made an accessibility plugin for the Unity engine that fills this gap:
https://assetstore.unity.com/packages/tools/gui/ui-accessibility-plugin-uap-87935
You can recognise that a game uses Unity by the fact when it launches, it's as if VoiceOver goes to sleep, and tapping random spots on the screen might eventually cause something to activate. If this is the case, when emailing a developer of such an app, explain that because they used Unity to make the app VoiceOver (as well as Talkback on Android for that matter) can't access it, but with the accessibility plugin and similar work as described above with labeling buttons, it might be possible to greatly improve the situation. Don't forget to then include the link to the unity asset store that I gave above.

This turned out quite a bit longer than I thought, but hopefully it'll help people out looking at reaching out to developers!

If the app is somewhat accessible, look in settings or info inside the app. If completely inaccessible, in the app store description, look for Developer Website Button or it can be labeled as contact the developer or similar language. It can be helpful to include the urls in the previous post with your message.

Hello,
There is a section in this AppleVis Site which gives out information to developers about making apps for the Mac and iOS accessible. Here it is:
https://applevis.com/developers
In your reports, feedbacks, or when you contact developers about apps, regarding accessibility, you may choose to include this above link. It lists very useful resources. Also, since more and more apps are being developed using Web based engines, you may also include the below link in your feedback, which is a video from the World Wide Web Consortium about making content on the Web fully accessible:
https://www.w3.org/WAI/videos/standards-and-benefits/
I hope this is useful to you, and keep persisting in your advocacy efforts until every developer is aware about accessibility!

I just explain about voiceover
ex. I use your app with voiceover. If you don't know what that is, it reads any items to me that are on the screen on my apple device.

I also give them this web site and something like if you have any questions about voiceover or how to make your apps accessible you can go to www.applevis.com. they have info on how to make apps accessible as well as more info on voiceover.
Also tell them to contcat you if they have questions about Vo.