Manuals, lots of manuals, recognition publications, intelligence bulletins, reports and so on.
From a US perspective some of these were not necessarily accurate circa 1941-1942, but as time went on, well, the more information that comes to hand, the better the presentation. This was a lot of effort gleaned from pre-war measures such as reports from military and naval attachés, reports and information from Allied Powers, field intelligence activities, examination of captured documents, materiel, and equipment. Thirst for information on the enemy was insatiable.
See for example:
Early war identification manuals
Italian aircraft - http://ibiblio.org/hyperwar/NHC/NewPDFs/USArmy/US%20Army%20Field%20Manuals/USArmy,%20Basic%20FM%20Mil%20Intell.%20ID.%20Ital.%20Aircraft%20FM%2030-39%201941-10-24.pdf
Japanese Naval Vessels –
Armored Vehicles, German, Japanese, Russian, Italian -
British Armored Vehicles –
You can find these and German, Japanese, Soviet, and, yes, US aircraft manuals here -
Then there is the Journal of Recognition, some of the monthly issues can be found here http://ibiblio.org/hyperwar/NHC/NewPDFs/USA/Journal%20of%20Recognition/ for example the September 1943 edition (big file, takes a while to load) - http://ibiblio.org/hyperwar/NHC/NewPDFs/USA/Journal%20of%20Recognition/Journal%20of%20Recognition%2043-09.pdf
Some US Navy ONI Recognition Manuals can be found here
for example, ONI 41-12 Japanese Naval Vessels - http://ibiblio.org/hyperwar/NHC/NewPDFs/USN/ONI%20Recognition%20Manuals%20and%20other%20material/ONI%2041-42.Japanese.Naval.Vessels.pdf
Or, perhaps, “Japanese Operational Aircraft – Know Your Enemy,” a CinCPac/CinCPOA bulletin from April 1945 http://www.ibiblio.org/hyperwar/NHC/NewPDFs/USN/ONI%20Recognition%20Manuals%20and%20other%20material/USN%20ONI.Know.Your.Enemy.Japanese.Operational.Aircraft.1945.04.pdf
The Military Intelligence Service of the US War Department published an intelligence bulletin that provided information and photographs on enemy equipment. Here is volume 1, No 1: http://www.ibiblio.org/hyperwar/NHC/NewPDFs/USArmy/USArmy%20MID%20Intelligence%20Bulletins/USArmy%20MID%20Intelligence%20Bulletin%20Vol.%201%20No.%2001%201942-09-00.pdf. Volumes 1, 2, and 3 may be found here http://www.ibiblio.org/hyperwar/NHC/NewPDFs/USArmy/USArmy%20MID%20Intelligence%20Bulletins/.
And there are the USAAF Intelligence Bulletins
And if one’s military attaches at the time were clever, there were also small insights into who was who in the opposition. For example officer registers for the US Army provided back ground on serving officers, dates of rank, qualifications, service schools see https://archive.org/details/officialarmyregi1941unit/page/n5/mode/2up?q=United+States+Army+Register+1941 . And the USN, up until 1941 provided current assignments in both directories and registers of officers: Register for 1940 http://www.ibiblio.org/hyperwar/AMH/USN/Naval_Registers/1940.pdf ; Navy Directory 1941 https://ia801305.us.archive.org/21/items/navydirectoryoff1941unit/navydirectoryoff1941unit.pdf
One would only have to go down to the GPO and put money on the counter . . . these were not classified.
Everybody did it. Some carefully worded searches can give you all kinds of things “Australian Army Primary Documents” will get you here https://www.army.gov.au/our-heritage/history/primary-materials which will get you here https://www.army.gov.au/our-heritage/history/primary-materials/1939-1945-world-war-two and if you scroll down to the bottom of the page, the next to last item Pocket Notes on the Japanese Army https://www.army.gov.au/sites/default/files/2019-11/pocket_notes_on_the_japanese_army_organisation_and_equipment_1944_0.pdf
A search for Royal Australian Navy Primary Materials will get you here, https://www.navy.gov.au/media-room/publications which will get you here - Combined Operational Intelligence Centre Naval Summaries, which will get you https://www.navy.gov.au/media-room/publications/wwii-combined-operational-intelligence-centre-naval-summaries, the Combined Operational Centre Naval Summaries, and picking the first one and snooping to page 13, we find for 5/21-41 noted: “French Submarines in Indian Ocean - C. in C. East Indies report's ‘Censorship of mail from French Ship ‘SONTAY’ reveals following - French submarines ‘ELONGENS’, ‘ESPOIRE’, ‘MOUGE’ and one other accompanied by tanker which victualled and fueled them at sea reaching Tamatave nonstop from Dakar about 16th January. 2 have left for Indo-China and 2 are remaining Tamatave. Graded A.I. Name of tanker is "LOT” and it is presumed she will be accompanying submarines to Indo-China.”
Or from the same base publication page you can get to the Royal Australian Navy Monthly Naval Warfare Review at https://www.navy.gov.au/media-room/publications/acb-0254454-royal-australian-navy-monthly-naval-warfare-review-wwii for contemporary documents on goings on and intel/recognition information.
Documentation could be fairly simplistic, even crude, for example, these descriptions of Japanese ships distributed at NAS Maui in early 1943 (from my collection, this is page 5, the start of Japanese warship descriptions, the first four pages were descriptions of US warships)
Others fairly detailed and esoteric, such as these examples regarding the Arado 234 (both also from my collection; the first from a 28 page document entitled “German Arado 234 Bomber – A Collection of Data from Various Sources” and the second, obviously the cover of a second document of 30 pages)
I am leaving out a great many internet places which require a fee to access their available documents (yes, I avoid them, too), and sites requiring academic connections which I do not possess to access same. But, Intelligence, everybody did it. Everyone wanted to know what those people was doing with what equipment and what our guys were doing about it.