Yeah those are the kinds of things I still have to partially figure out. I have no writing experience, only an active imagination. It would take a lot of planning and conceptualizing to get that setting/story into a shape i'd be happy with.
1 - How strong is magic? How much of the effects of our own modern technology could be replicated using it? If history is anything to go on, new lines of development start when the current designs reach an apex of development. If you couldn't magic up food, it's perfectly possible that we'd have still had the agricultural revolution of the 16th-19th Centuries, which in turn would have led indirectly to the growth of modern chemistry (ie fertilizers) in the 20th. So it's possible that we could have a lop-sided world scientifically; we might have plastics and jet engines, but no penicillin or antiseptics because the 'Healer's Caste' always managed to do fine without them.
Items can't be apparitioned into existence with magic. Things like trains/cars and other advanced technology were invented by magical mechanics. Basically technology is as advanced as it is for us now, only decades in the comparative past. Magic sped up our invention processes. So they have computers for instance, but with old fashioned design sensibilities. Mages can teleport but it's not as cost effective as regular transportation, there's large train networks, both above and under ground as well as roads and highways with cars. Magic requires bodily energy, which restricts the 'magic can do anything!' idea you have in Harry Potter. Teleportation would be an advanced technique for emergencies/special occasions.
There is indeed a healing caste, but there is no 'healing magic'. They can't just wave their hand and heal someone. Wounds still need to be treated as they are in real-life. And the ability to manipulate physics come in there. So there's hospitals where there are doctors who just do modern day medicine simply with different, magical, tools. They are by the way the only ones to use wands. Staves are the main magical apparatus but they usually need smaller instruments. So things like incisions are made by wands, and they use wands to perform surgeries etc.
2 - How hard is it to learn / how many people have the 'ability' to learn? If only 10% are mages, and you say a gun wouldn't work against them, then firearms would still work against the other 90%. If the teaching of magic is really restricted and/or hard, you'd end up with some clever Muggles inventing stuff to do what mages could do anyway. You also might end up with a situation where a mage could rustle up a fireball to kill you, but it's just damn easier to pull out their flintlock musket and shoot you in the head. I seriously doubt Muggles will simply stay in the Dung Ages scientifically unless literally forced to. Even if most people can use magic to some degree, you'd need tech/enchanted shit to help out those who aren't very good at stuff.
3 - The cream rises to the top. If magical ability was hereditary, you'd end up with the majority of the upper classes mages and ability in it a fast-track to position of wealth and command. This would cause society stress, with possible apartheid-esque divisions between the mages and Muggles. If it was on the basis of intelligence, you'd end up with a more Brave New World thing - they'd bound to tinker with Eugenics to try to breed more magic-users.
Just off the top of my head.
The people who don't have magical abilities are basically peasants with their own rights, which they aquired in their emancipatory movement about a century prior to the story I would write, with indeed the upper class being basically all magically gifted. The power mostly lies in wealthy mage families which take part in the governments and politics. Though the 'muggles' still have their part in the governments, however small. In the latest century and some centuries before the story takes place questions of morality and inclusion were raised and better rights were demanded by and given to the 'muggle' castes. The 'muggles' are part of the great industries and economies that make every-day items. Mages can also construct products, invent and produce technology etc. They are the higher caste of worksmen who create computers/most cars etc.
Since the mages do all the fighting, and the 'muggles' are not eager to fight against people who have magical abilities when they have none. Sure they could invent weapons and do some damage, but there's no need at all. They would mostly be cannon fodder, and that's an old practice no longer used in civilized times.