What does the daily life as a 91E3 “machinist” look like?

91E is actually a "Allied Trade Specialist" - (GoArmy.com description) As an Allied Trade Specialist, you’ll supervise the fabrication, repair, and modification of metallic and nonmetallic parts to ensure the Army’s equipment is operating reliably. You’ll operate lathes, drilling presses, grinders, welding machines, and other machine shop equipment while also serving as a welder. You will train to select the materials, tools, and equipment to do the job, and learn how to operate fabrication machines.

Good link that describes the training for it.

https://www.army.mil/article/92340/credentialing_program_helps_service_members_excel

I found this on Army Writer:

91E30 Machinist
Fabricates, repairs, and modifies metallic and nonmetallic parts, utilizing such machines as engine lathes, utility grinders, power cutoff saws, armature undercutters, arbor and hydraulic presses, drill presses, and their associated attachments, accessories, and tools. Supervises lower grade soldiers and provides technical guidance to the soldiers in the accomplishment of their duties.

Kinda think life would be in the motor pool which, I wasn't that fond of.

It means you will make a very good living when you get out. Make sure you keep a copy of every injury you get because you never know what will happen later on in life.