Pretty much what Dave said, and I reckon they were similar all over the world except maybe some hot countries they were just under a shady tree.
My late Mother, born 1922, grew up in Grandfather's Garage and told me the same as Dave says.
Personal property of the employee mechanics was only basic hand tools, usually in a wooden home made box or carry tote. Some special tools like bearing pullers were owned by the business and hung on boards or on the walls of the building.
The building itself was an uninsulated corrugated iron shed at ambient temperature, roasting hot in summer and freezing in winter with condensation dripping in between!
The facilities for workers were very basic. They ate packed lunch in the workshop. Toilet was a shed out the back. Cold running water was considered a luxury, but they did have that. Everyone was covered with oil, filth and grime and just lived with it. My Grandmother said doing the laundry was a nightmare as Grandfathers hands were always covered in waste oil and this contaminated his underwear every time he went for a pee. She said cancer growths on the scrotum was common because of this, but Grandfather survived that and 2 world wars but died of bowel cancer anyway.
The cars and trucks of that era required a lot of maintenance compared to vehicles of today, valve seat grinding every 30,00 miles, but as Dave says this was all basic stuff and done on benches in the workshop.
Whilst WW2 was going on of course they were all terrified of air raids and bombing and listened out for enemy aircraft all the time. The gasoline storage tanks underneath the forecourt would have blown them all sky high had they ignited. The nazis did aim an incendary bomb at the Garage, but it missed and landed in Great Uncles haystack nearby , shame, it burned all his hay. They spent the next decade joking about it, saying things like: "What is hidden in your haystacks, to make them a strategic target"?
Fuel was rationed of course (I still have some coupons) so in desperately short supply. Used lubricating oil was saved up from use in cars and re used in our farm tractors it was so difficult to get, and straining it through a cloth stretched over a funnel was a messy job.
That's what they said it was like.