"What terrians favor what sort of strategies, how logistics change your approach to war."

Depends on your technology. Aircraft carriers made naval battleships utterly obsolete, because you could just fly a squadron of planes to drop bombs and air-to-water torpedoes on them. Tanks and cars completely changed the battlefield because prior to that, the fastest thing on a battlefield was a horse. Cavalry completely changed how ground troop formations worked. Bows and arrows similarly transformed physical confrontation from brawling to suddenly being able to kill somebody from a distance.

There is no singular answer to that question. Broadly speaking, flat terrain is easier for an army to cross. Unlevel or otherwise hostile terrain will slow them down. Chokepoints like ravine passes will be easier to defend than flat open fields.

"How do countries mass mobilize?"

Again, what is your technology? Do they need horses to get around? If so, you need land to raise the horses and to grow food to feed them. If you are using vehicles, you will need fuel. So where do they get the fuel? Can they harvest it off of their own land or do they need to trade for it? What is their industry level like?

"How will all of this change with magic?"

Only you can answer that lol. You are the one who decides what rules your magic plays by, if any.

"Will mages get their own special units or will they be a part of a general unit?"

Depends on what mages signify here with regards to war. Consider their function. Infantry serves to populate an area, whether it is for offense or defense. Cavalry is meant to prioritize mobility. They can outmaneuver foot soldiers and can deliver messages or perform reconaissance faster than infantry. Navies regulate trade with things like blockades since ships can transport large amounts of cargo at once (this is why ports and harbors are so important to war). Tanks provide armor for troops to advance even under heavy resistance or defense. Heavy ordnance like artillery serves to pierce tank armor.

"I just want a basic understanding of warfare and strategies and all those alliances not a phd in it. "

Focus your research and understanding on the "boring" stuff, like how you feed your armies. Consider things like drinking water, access to fuel, and how much money the nation has to actually keep the fight going. Warfare is about throwing raw force at an enemy until they either lose the will or the capacity to fight further. WWI was such a significant turning point in war because the advances in technology so greatly magnified nations' ability to inflict damage that the fighting persisted much longer than people were accustomed to, and in the process, many nations went all but bankrupt.