but in this world the goddess adds a special buff kind of divinity+ which counters the demon's god's buff, this is explained in the next chapter or couple of chapters at the current manga's pace. As for "who" or "what" they pull magic from, in the world they're currently in it all comes from very inefficient incantations and incorrect/absurd verbosity that call upon the world's entities that hold power on specific magics, but the world is so backwaters and underdeveloped that they don't even understand this, and as such they never developed much in this area, only using inefficient magics empowered by the goddess's blessing of elements; on Suimei's world, this has been vastly improved, with lots of I+D+i pourd on discovering where magic comes from and how to invoke it as efficiently as possible with power outputs required for very specific things and/or spells, they deconstruct it so much that even "gods" from which people pulled power from have become mere formulaic words to set into a magic formula, as well as stars, planet positioning, etc. this goes into such development that each magic incantation is especially tweaked depending on your location on the planet, and the movements of rotation and translation of the earth to know exact star system positions and other planets and/or celestial bodies.
Suimei actually has many problems to use his magic correctly since the stars aren't the same, nor does this world have the same beliefs and other stuff as his own, which makes invoking specific magic systems very complicated (a magic system is something like the kabalah system, or the solomonic one). In the end, even with this disadvantage, Suimei has absurd firepower for this world, he's able to stop fighter jets, tanks, and even carpet bombings without as much as breaking a sweat, he has a very hard time pulling the magic system's power efficiently and even then, he's able to basically disintegrate demons simply saying that they seem to be quite durable for being a bunch of slow, stupid and disorganized mass of boring targets, which he seems to enjoy as he can experiment quite well with them since they won't break easily. Oh, and he does mention he is but a middleground magician and nothing special.
The LN explains this and more, to the point that the current world in which they are has magic equivalent to a fireplace vs a multi-nuclear reactor station to produce energy, the level of complexity is orders of magnitude apart.