How to automatically run apps in iphone?
Using the example in your question, if you're wanting this to happen automatically without some sort of user intervention, then I doubt this is going to be possible. This is because of security restrictions in place to protect your privacy, data, etc. For example, being able to open the YouTube app would require the iPhone being unlocked, so it's not something that can be set to just happen at 8am every day with or without you.

If you're talking about a way to quickly do this sort of thing with a single tap of a button, then you may want to check out Apple's Workflow app.

It does take a bit of exploring and getting used to, but once you create a workflow it can be launched from the Today Widget or even be exported as a standalone app on your home screen, amongst other options.

I don't think the exact example you give in your question would be possible, at least not yet, but Apple is working very hard on increasing the functionality of both Siri and Workflow to incorporate better integration within and between apps. It's a case of 'watch this space'.

But, even with the current version, I've seen people achieve some pretty cool workflows, so it may be possible to do what you want if you find the right app. By this I mean that there's a Workflow Action Extension that allows workflows to be run from other apps - so if you could find an alarm app that uses this extension I suppose in theory you could use that to trigger your workflow automatically because it'd be an alarm going off.

Finally, if the example you gave in your question is actually all you're wanting to achieve, there are many 3rd party alarm apps that play YouTube videos at the allocated time. You could check to see if any of these will play whatever video is first on the YouTube list.