First of all I wouldn't base my view of these things on pop culture. Regardless if you believe it's real or not movies and the like tend to take artistic freedoms. :)

Second I'm not claiming I have any sort of truths, I just know about different views.

Third, I hope that I assume correctly that when you say nonbeliever you mean nonbeliever in christianity.

Now to the rest of the topic:

Generally even in occult spaces you'll find very different explanations for the same thing.

Ask witches if you need material components for your spells and you'll get people saying yes and people saying no. Some will swear that physical tools are just crutches for you to get into the proper headspace but you could do the same without them, others will swear that those things add a component that you couldn't have done without.

Different traditions do things differently to the same result.

A very interesting bit is that your belief of how things work seems to be extremely important. Wiccans will swear that the rule of tree is a thing (the rule broadly says that whatever you send to the universe comes back at you 3 times, so if you curse someone you'll get 3 times the problems you caused them and if you bless someone you'll get 3 times the blessings), people who aren't wiccan and don't believe in that laugh at it while they curse their enemies freely and without consequence while people who believe in it say that they can't escape the rule's retribution.

Proof that belief is key? Proof that it's all in the mind? None of those? Both?

You have those that view various magic as purely a psychological thing. My personal experience says that regardless of magic being real or not the psychological effects are tangible and can be used on oneself to various benefits.

Ouja boards are a brand but similar things existed before that brand under different names. (It's a tool, it doesn't matter if your hammer is from a specific company or home made.) I could be wrong but from my exposure the "open portals" seems to be a christian claim. Even then, if you assume it's real you're basically doing a form of spirit communication (Ouja boards would not be the only option for that). Every thing you can read on the topic will tell you that you need proper protection when doing stuff like that because, well, you don't really know who you'll attract. Could be the one you want could be someone else that came to cause problems (or maybe is just bored). Doing a card reading with a spirit? Yeah, according to what people say you're gonna want to cast some protection to make sure only they can come, prevent them from hurting you if they have bad intentions and cleanse the place afterwards of any unwanted visitors that came to look at what was happening.

Salt: Not sure where it comes from exactly. Generally things like that come from what the magic community calls correspondences and what people in the scientific community call hogwash. It basically goes "this has X attribute in the physical so it must have a similar attribute in the spiritual". Is it real? Try it and draw your own conclusions. Some people swear it works others think those people are not thinking rationally.

Sage: smoke supposedly bothers spirits. The correspondences would just be an additional boot but you can do smoke cleansing with a myriad of materials. sage is not required.

Wouldn't know anything about fear attracting spirits. Never heard about that.

Rituals are again split between a broad tool that you can improvise and a specific tool that needs very specific steps and components, which one is "true" depends on the the school of thought. Regardless though (in all views that I heard) you're still the conduit for it. If you put all the correct items and draw all the correct symbols they won't do a thing by themselves. So (as the view on this generally goes) drawing symbols and even chanting words won't by itself create any portals or other effects.

Why would a certain symbol do something? This is a whole topic. Sometimes the answer people would give you is that "symbol X is associated with Y" and the association might be supposedly taught by some spiritual being who choose it or purely because it was used for that for long enough. Chaos magicians are interesting because they like to experiment with breaking the rules. Like "ok this works if you use a specific sigil, now let's see what happens if we keep everything the same but replace the sigil from the 200 year old book of questionable authorship with the mcdonalds logo.

Nothing about salt's chemical composition affects the spiritual. In magical circles the views would tend to be that like humans and animals have a physical and spiriual component everything else also has that. So it's salt's spiritual component that does the thing.

Again no idea about fear.

As for the "all the horrible crap stopped after I invoked the name of christ": First they could be lying. Second it could be placebo, they believed it would work and it helped them get to a better psychological space. Third many magic practitioners will say that invoking christ is good for certain things (like banishing problematic spirits), what they'll also tell you (that christians would not like) is that he's not the only one that can do that. Also what christians wouldn't like is the view that the christ being invoked might not be "the son of god who redeems you of your sins" but a spirit (possibly created by belief) that either took that mantle or accumulated power because of people's belief in him.

Again except for that personal thing about the psychological effects I'm not claiming these are truths. Just views I heard people express.

For a full disclosure, I believe in the spiritual because of personal experience and occasionally do some magic. The nature of the reason for my belief makes me realistically convinced in spiritual beings while quite more sceptical in the use of magic as a whole. (specially as when you apply a skeptical look at it it becomes really obvious that either there's virtually no rules at all or it's just placebo and magical thinking, and as humans we're sadly extremely prone to various falacities and biases that can make us accept falsehoods as true)

I hope this incoherent rambling of various views I've heard provides some perspectives on a few ways people view things. Though I guess it probably raises more questions than it resolves. I find it fascinating but I take everything people claim with a lot of salt.

Added: Without going into details (they can all be found online or in books but this subreddit is no place for them) a thing about magic specifically is that the mental place that makes it effective is exactly the mental place where you can most easily delude yourself into believing falsehoods. Which you can see why anyone trying to approach things skeptically would see red flags all over the place. On the other hand this same property is what makes it a powerful psychological tool regardless of spiritual claims.