I do recommend the stormlight short stories, they are both quite good.

What info are you specifically looking for?

*grunts* I should not have read SA so fast (according to the receipt, it's exactly month and two days).

Thank you both @SkybreakerEngineer@lemmy.world and @Nighed@sffa.community for the explanations.
I feel like one of the ten fools!

I will have to read it all again. Seems I've missed some key details in both MB and SA as I have hard times connecting some of the stuff you have mentioned to what I am pretty sure I should already know.
Also should have read SA in my native language.
And and should have read Mistborn: Secret History ...

On the bright side, now I don't have to worry what to read (the whole) next year, will probably wait for WaT to release in December and start with everything again. Thanks again though!

The more of the Cosmere you read, the more things will connect and the clearer the picture will be. Every time I re-read SA, I see new connections I'd missed before. Some of that is familiarity with the magics of other planets, because that changes your read from "character did a weird thing" to "hey! that's magic from this other planet; why/how has it turned up here?"

In the earlier books, the crossovers between worlds/magics and the underlying "how things work" are more subtle and you'll miss things on first read. In more recent books, it's more overt.

Some of that is because of how much the protagonists themselves understand. For example, in the first Mistborn trilogy the characters really don't understand what's going on on their own planet, so of course you don't get a good explanation. In Secret History, the POV character does run into people who know quite a lot about what is going on, so when Secret History revisits the events of the main trilogy you're able to understand the forces driving those catastrophic events.

The characters in SA started off thinking magic wasn't real and knowing nothing about realms and worlds beyond their own. They are learning a lot through their spren and Hoid, but there is still a lot that they don't know. And you as the reader are learning along with them.

There are some fundamental concepts that span all of the systems. Investiture is the "mana" here but can show up as matter too, like the mists or shardblades. Investiture always wants to be used, and in a way that rewards people for satisfying the Intent of the shard it came from, like how radiants progress by personal growth and deepening the bond. When it's being used, it makes vibrations that can be picked up by Seekers and the like.
Magic systems in a particular area are affected by what's going on (or happened in the past) with the shards there. Like how the Dor works because 2 dead shards are corked up and trying to get out. The shards can also consciously affect the rules to an extent, like putting limiters on how much power the radiants get. There are a bunch of side stories set on worlds with no shards, there's still magic but it gets wacky because shards fought there once or something. There's also magic that Ado himself set up, that tends to be either subtle like Hoid's stories or stone singing, or bonkers megastructures, with no in between.
Most magic has a Connection to the world it came from, but people figure out how to bypass that and you get the Cosmere Space Age.
Connection is something you have already seen a lot of, in the form of Bondsmith shenanigans and to a lesser extent the wind runner lashings.
The Intent of the user also affects how the magic works, it's why you can use magic without realizing, or do really complex stuff without doing all the math yourself. It's usually bundled up with a Command, where you give a verbal order and your Intent kind of gives it flavor.
The fact that different metals do different things shows up a lot, but it's not 100% consistent what each metal does.
And then there's the 3 realms. You've seen how the first 2 work, and that whole scene at the end of RoW is them looking into the third. The third is also where souls are (including the bits that govern memories and what magic you can use), Investiture returns there after it's been used, and all of time and space are one there. There's no alternate timeliness or time travel, but you can see the branching possibilities for the future, or play "what if" games like burning electrum. When something is looking at all of that for you and handing you the TLDR, it's called Fortune.

This is my very rough back of the napkin explanation but I’ll give it a try.

There are 3 aspects of the world. The physical, the spiritual and the cognitive.

The one thing all of the magic systems have in common is that the different shards affect how these different realms are used to affect others. Mainly through investiture (the spiritual potential energy if you will) to affect the physical, cognitive and spiritual realms.

In Roshar, for example. Investiture allows some to specify intent into the cognitive realm. Where the mental perceptions of things exist and have a kind of will over time. And when those are convinced to change it impacts the physical realm. The energy source allowing that intent to be work is Stormlight.

While in Scadrial (from Mistborn) there are actually multiple magic systems. One of which allows consumption of some aspect of the physical realm to gain enough investiture to affect the cognitive realm/spiritual realm to affect the physical (by changing various forces of attraction, repulsion, etc). That’s not even including Metallurgy which is its own thing.

I definitely recommend reading the Mistborn series where you’ll see his magic system from a different perspective and start to see parallels.

But at the end of the day there’s

• Mental will or intent by somebody with a link or “investiture” to a worlds magic system.

• The things to be changed/affected in one or more of the planes of existence by using the spiritual investiture

• The investiture source usually from consuming something from one of those planes such as metals, life force, cognitive.

For example I think the Nightwatcher tends to impose some kind of cognitive cost for her boon which affects spiritual bonds. So even in Roshar you have the impact of multiple shards and can see how they tend to warp how the magic works.