My understanding of the magic in Frieren is that it's all based off of "visualization" or "imagination"
Basically a mage has to imagine their mana manifesting in a specific way to cast spells and their limits are their minds/imaginations. For example, a mage who specializes in water magic, like that one dyke, has to imagine her mana manipulating the water in her environment to achieve a specific end goal, such as being balled up and launched at a dude full force. Frieren said herself, "can you imagine defeating a water-specialized mage in the rain? I can't." This is because the water mage is at such a high advantage there is literally no way to manifest your own mana to counter it. Similarly, this is what makes a mage like ubel so threatening. She's so retarded that there are no actual limits for her because her brain doesnt imagine something can't be cut. When that test happened and the other mage visualized nothing cutting through his cloak, every other mage was unable to do so either until ubels retarded brain just said, "but that's just cloth tho lol" and so she manifested her magic to cut through his cloth and body. She's basically the bottom end of the bell curve.
Now, coming to zoltrakk and freiren, zoltrakk is the purest form of magic because it is essentially imagining the basic form of mana manifesting as itself in the form of a laser. Immediately it got adapted into a sheild form. But the idea is that it's pure mana being launched rather than the more complicated extra steps of making your mana manipulate the rocks and stuff around you. However the important things about this visualization is that there are 3 parts to it. The beginning, which is just the current state, the middle, which is what the mana is manifesting as, and the end, which is the result of said spell. In ubels case for example, it's
guy has a cloak > my mana can cut that cloak cause it's just fabric > guy is cut in half.