As Germany doing Blitzkrieg I have formations with just regular infantry, formations with just mechanized infantry (alternatively motorised, but either will work) and formations with just armor.
The regular infantry are used to guard the length of the front so the enemy doesn't push forward into territory I already hold, to help the armor with the breakthroughs, to move forward into the breach and guard it once I've made a breakthrough, and they are the major force used to squash the pockets.
The mechanized/motorised infantry formations are used to help making the initial breakthrough, to follow up a little behind the armor and guard their rear and flanks, and also to move forward and help them if they have trouble taking a province that has difficult terrain for armor (like urban locations).
The armor are used to break through, push forward and quickly cut off enemy troops in the front, and then just keep pushing while the regular infantry kills the pockets that have been formed (alternatively help the infantry, but it's better to keep pushing forward in my opinion, especially if enemy resistance behind their lines is light).
Two things are very important:
When making a breakthrough, the fast divisions must be given orders to attack and move on to the next province behind the province being attacked so they can keep moving forward right away instead of having to wait for 24 hours before they can be given orders again.
And the fast formations (the armor and the mechanized/motorised formations) must fan out as much as possible when moving forward after a breakthrough rather than moving along a narrow line. This makes it easier to encircle the enemy and, most importantly, makes it more difficult to cut of your armor spearheads. Also, if you can widen the initial breach to be more than just one province wide with your regular infantry, then do that as soon as possible.
When attacking the Soviet Union I make 2 large breakthroughs in the initial phase of the invasion and form 3 pockets. The northern pocket is formed by reaching the sea near Riga. The central pocket by going around the swampy terrain on both sides (north and south) and meeting east of those provinces, where you have open terrain again, to close it. And the southern pocket is formed by pushing southwards to the black sea.