Norman Smith was their first Engineer thru Rubber Soul.
Geoff Emerick actually did a special remastering of Sgt Pepper for its 30th anniversary...and somehow I have a copy of that unreleased version. Definitely more clear & detailed than the originally mastering.
A Beatles Historian did the best version of documenting their recordings. His last name begins with “M” - can’t recall his name...
Ok, the book is by Emerick. Go to Amazon. It is definitely worth reading, and I’ve read most related to the Beatles.
Mark Lewisohn