How can I find WWII info on what Company and Division my grandfather was in?

How can I find WWII info on what Company? (A-B-C) or Division (infantry?) my grandfather was in?

On Musters of 4-30-1939,  it's Company K, 29th Infantry. But 4 months later on Muster dated 8-31-1939, it says Co. B, 38th Infantry.

Most confusing is the Army - Army Air Corp division & "searches" and he actually retired from the military at Vance Air Force Base, so he's shown as the Air Force also.

So many moving variables.

I want to see if I can possibly pinpoint him overseas - Normandy, Northern France, Rhineland, Ardennes & Central Europe (which I know from his medals he received) but I'm confused as to what search "words" to use to drill down to facts.

His name was Glenn Hardgrave, retired Master Sergeant and his Army serial No. was 06260307.