To be honest, these days most people don’t use individual smartapps, if they want more complex custom logic they just use Webcore. See the following FAQ. (The topic title is a clickable link.)
Between that and the custom automation creator that’s built into the smartthings app, that’s all most people need. 
If they ask a question in the forum and someone recommends a specific smartapp, then there will be instructions for how to find that.
There is also a popular library of very sophisticated custom smartapps behind a paywall at
You pay a one time fee and then you get access to everything in that library. And @Rboy does a lot of work keeping the code up-to-date, so I believe everything will work with the new V3 app, but you can ask them if you want to be sure about a specific item. Again, very popular if you need any of those advanced features.
Other than that, there is a quick browse article in the community created wiki that divides out smartapps by topic. That was very helpful up through 2019, but again, now most people just use webcore. But here’s the link to the quick search if you want to take a look at it.