The question you have asked would easily fill several textbooks to answer. Fortunately, said textbooks are already written and readily available through a number of sources.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

I've been involved with what you called a heavy fabrications. I worked at a 4-wheel drive articulating Ag tractor plant. I would not call those fabrication a heavy application. The main side plates of the frame were .500" thick for the large model. The front and rear frames weighed about 4000 pounds. For a heavy fabrication think of a submarine fabricated out of 2" thick or thicker 80k yield steel with full penetration welds welded into a single hull 400 feet in length. That is a large fabrication but of course the ship yard I am familiar with alse built Nimitz class aircraft carriers over 1000 feet long.

I have also worked at a large(heavy) job shop with one specialty was tire molds for 8-10 feet in diameter tires. The tire mold castings were cast at the company into the outer case and the inner tread ring. The outer case was machined in the inside and the inner tread ring was machined on the outside and then these pieces were pressed to together with an interference fit.

The machining process and the parts are massive but the biggest problem was material handling. When handling a part which weighs 4 to 20 tons a single mistake will kill you.

Lot sizes were very small. Cycle times were sometimes very long. Think of a high speed steel gear hob 12-14" OD cutting an 96" OD gear with a 10" length of the tooth. First roughing pass starts and the next day you check the progress of the first pass. Cycle time measured in days.

If you want to designate the processes for these parts start talking with the old timers and become very good friends and they will help.

I worked in a plant that had the capability to roll 12" plate - quite the sight.  One vessel we manufactured spanned 11 rail flat cars when it shipped.  The rail car bodies we manufactured were the light fill-in work.

rmw