Linux

Linux is fairly secure out the box and typically does not need any sort of extra hardening for most people unless you have a specific case you are worried about or some threat model that requires it. And hardening a system is not simply about installing some package, but more about learning to setup and utilise said packages to mitigate the threats you think you are going to be dealing with. Hardening a system generally comes with tradeoffs and these are not always worth the cost involved for what you get from them. All depends on what types of threats you think you will face - a journalist in a hostile country is going to want a far more secure system and will be more willing to compromise on other aspects to get that then some grandma that just wants to look at pictures on facebook. Both of these will want different tradeoffs for their systems.

Generally speaking I would start by reading up more about hardening linux systems, and what types of things these tools are designed to do. I would start with anything related to the system you are interested in, nixos has its own guides general security which links to many things you might want to think about. Arch Linux also has some good guides on security that are worth a read. And there is more general stuff like The Practical Linux Hardening Guide or redhats guides though these are more server focused and might offer tips that can be too restrictive for desktop systems.

As for apparmor and selinux, these are competing technologies and I don't think you can use both at once.