Spirituality is how we make sense of the unknown and unknowable. When things happen to us that don’t make sense and we can’t derive meaning from, spirituality steps in to fill the gaps.
It can be a fully-individual process, humans can if they really really need to come up with a spiritual ‘framework’ all by themselves, but in general humans get their spiritual guidance from, well, just about anywhere, from stories they hear, movies they watch, books they read, sermons they manage not to sleep through should they be lucky or unlucky enough to go to church, anywhere really.
This spiritual guidance is super important when we’re growing up and remains important throughout our lives, in this respect it matters not what one’s position on non-physical reality is. Everyone has philosophical ideas they cherish or value at least understand to be worthwhile, these count just as much as what we’d call spiritual or theological.
To arrive at the spiritual, start with the material and work outward. Matter operates by deterministic rules that we understand quite a lot of, not everything of course. Those rules, which we can write down in books, are of matter, but they’re not themselves matter. They’re abstract.
Abstractions are how you move outward. You can abstract outward from a person’s material body to the person, including things like their ideas and friendships and yearnings, things that are part of the person but not of their body. Spirituality is how we conceive of and work with these abstractions. We think about them, and the rules and stuff we derive from our ideas and the things that happen to us and what we want to happen, become part of our spiritual selves.
Anything that operates by rules that are knowable and understandable, is less spiritual than things that operate by rules that are obscure and unknowable. Humans spend much of their creative energy dealing with the things they have a hard time understanding. And creativity is one of the highest forms of spirituality imaginable.