Flutter/Dart (or perhaps a game engine Unity/Godot) for mobile unless there's specific support that I need more natively.

Actual backend varies depending on the project and can be anything from Go, F#, Kotlin/Javalin/JDBI, Elixir/Phoenix, and an SQL database either PostgreSQL or MySQL, sometimes CockroachDB, or a service like Firebase. TBH, the choice of backend tech doesn't actually matter that much once your application has a working foundation. Adding more usually just means replicating whatever, hopefully good patterns you've already created, and each successive addition gets easier as the foundation or examples to copy from get larger. Even a PHP/Yii project was as manageable as Ruby/Rails if it's rolling and cared-for, but I wouldn't personally choose these as I prefer static typing and don't feel slowed down by it.

Don't have an answer for Desktop, or need. If I did, I'd want something direct and cross-platform, maybe Java FX (or now Flutter).