Mutts are versatile but need investment to shine. They get to choose and upgrade one set of armor/infantry/air power abilities separate from their doctrine choices, their halftracks can upgrade into anti-infantry, anti-tank, or ambulances and every single unit gets to choose 1 out of 2 abilities when they gain veterancy. Riflemen, for example, can choose an active ability to suppress enemies OR another one that allows infantry to dash to the piece of cover you select and reduce incoming damage in the process. On that note, riflemen can be upgraded with BARs twice so they stay relevant mid and then late game.
Infantry comes from the barracks for rifles and the weapon support center for the heavy weapons.
Mutt vehicles are also all-purpose, although each tier has one vehicle that leans more towards AT and one towards AP.
Germany is full of tough but expensive specialists and they have a mechanic where you can pay to trade basic grenadier squads for more elite units, which is cheaper but loses you veterancy. The Grenadiers themselves have a lot of guys per squad but practically no upgrades other than submachineguns and whatever your field marshal provides, while the elite infantry is specialized and smaller in numbers. Field marshals are like mini-doctrines that provide bonuses to infantry, vehicles, and bunkers so you are kind of gimped until you choose one, and can be roughly divided into sneaky shit, vehicle stuff, and defensive play.
Germany tiers are expensive. You have to build a building and then buy an upgrade on top to access the heavier units that a building can make. Because everything is expensive you have ways to increase income too, like placing trucks on points.
Vehicles are all specialists as well so you have to choose carefully but they often have powerful upgrades that let them do fancy shit like see enemy icons through the fog of war, camo netting, etc.