Getting a ‘service number’ is critical and then finding the regiment.
It’s never a ‘one stop shop’ for most individuals’ records – some regiments have their own records, some are in the Nat. Archives at Kew and some were destroyed during WW2/Blitz.
In Kew the records are spread across several sectors, e.g. Pay and pensions in one bank of files, transfers in another, etc. Dates also are a factor, not all are in Kew
Some regiments have their own histories, written by retired officers, often very good for background info. Also worth looking at are the ‘regimental day books’ which (when available) are a goldmine of info on battles/killed in action, etc.
Find my Past has free access to military records until tomorrow (12th) morning at 10.
I’ve done some research in Kew and it is one area where – should I have to do similar again – I’d hire a pro. The records there are not intuitive but a pro with Kew experience would save you days of work. Kew probably is closed now due to Covid……..