Lore and Theories: Enchantment?
It’s well known that dwarves cannot be mages, and Tranquil are cut off from the Fade and therefore the source of their magic. And yet they are able to create magical items with runes, using the runes to enchant the chosen objects. (Not just weapons and armor as we see in DA:O and DA2, but also rings [the noble woman at the Winter Palace in WEWH misplaced a ring enchanted by the Formari and gifted by another highly-regarded noble], teapots [Celene has one for her tea], and presumably other items.)
The question is: how?
My headcanon answer lies within cymatics. (Please do check out the link because this is cool as hell, y’all.) To put it simply, sounds create vibrations, which in turn can create visible patterns in some media.
These patterns remind me quite a lot of not just runes, but the sigils we see in the games for various magical wards, traps, barriers, and even Mythal’s binding from drinking from the Well of Sorrows.
I’ve written before of the relationship between magic and music in Thedas - you could start here and then also read the linked post on the Fade and the Void, if you have the stamina. There’s a little lore in the canon (and further explored in various headcanons and meta in fandom) suggesting, basically, that the Veil is a vibration, and possibly the practice of magic itself is as well. Then there’s the song of the Old Gods that compels darkspawn, the importance of music and/or the Chant of Light to the Maker, and so on.
To get to the point and building off of the previous idea, I’m making a major supposition. Please bear with me.
It’s suggested throughout the games that before the Veil magic was a common thing that all elves (perhaps humans as well?) used. It’s also known that there had been contact between elves and dwarves pre-Veil. (Whether it was always bad contact, since the codexes regarding Titans paint a grim picture, I leave entirely up to opinion.) This has the potential for dwarves to have observed the use of magic in ways that elves could not conceive of because it came entirely naturally to them. It might be akin to singular experiences of the five senses versus synaesthesia, for example. Meaning, dwarves might have (pre-Veil!) seen and experienced magic entirely differently - as vibrations and color and pattern. (This might be entirely due to their connection to Titans/”the Stone.”)
Being able to analyze these patterns leads to establishing what patterns produce given magical effects. This pattern - or rune - creates fire, that one creates solid barriers, and a different one holds a target in place, and so on. Patterns, runes, sigils become both signifier and signified when imbued and crafted with lyrium. (Ceci vraiment un pipe!) Contact between dwarves and humans - we know Kal-Sharok had trade agreements with Tevinter, for example - would easily spread and share this knowledge, eventually allowing for the Tranquil to learn and enchant as well. (And thus break any monopolies on the magical item trade!)
Over the centuries, particularly with the interruption of civilization due to the darkspawn invasion, dwarves have lost the meaning for many runes. But for someone like Sandal, who we are told is a savant, this may not be necessary as his ability to craft enchantments may be purely instinctual, with a very strong tie to the Stone that allows him to still “see” magic.
This is precisely the kind of thing that Dagna would have studied and observed. And as her studies grew more complex, so would the runes and how their combinations into larger and more powerful spells, or amplified with various materials, would manifest - how to look out for accidental rune combinations that carry unintentional runes within, leading to unintended consequences (and potential explosions), and so on.
(The idea of rune combinations, by the way, is why I believe such things as Dorian and Alexius’s time magic was thought to not be possible: the kinds of rune combinations needed to make it possible without killing yourself or obliterating the Maker’s creation would be so incredibly complex as to require the equivalent of advanced mathematics with a ridiculous number of variables to juggle and control for. And some of y’all think Sudoku is hard.)
(Last aside: these runes and runic patterns is how I headcanon that rogues are able to disarm Paralysis wards in DA:O. The Circles of Magi have been teaching certain spells and rune patterns for centuries the exact same way. Thedosian thieves’ guilds and individual rogues would be crazy not to size that knowledge to practically avoid capture. A smear of lyrium on the finger or [blank] runestone used to change the pattern would disrupt and hence disarm it.)