Stainless storage tank fabrication

There are standards and standard details for large tanks as commonly used for water storage, petroleum storage, etc. I am not aware of any such standard for small tanks like this.
On the flange- I would expect either a corner-to-corner weld or the tank body inserted partway through the flange. If the fabricator had the equipment to do it, the flange could have a lip butted onto the body, or could be formed from the body.
I would expect the flange to be rolled to radius, or cut in one or more pieces from plate, and not turned on a lathe. As-fabricated roundness might not be perfect, but the cylinder it fits on won't be perfect, either.
I would expect holes to be drilled after forming, if rolled from bar. Normally, the two pieces would be clamped or tacked together and drilled together. If layout was by hand, it may not be possible to rotate the flange and still bolt it up.
I would say on each of these issues, it would be more important to fabricate something workable for you rather than trying to match the existing construction.
Talking to a fabricator that routinely does this type of work would be informative.
One big item is to confirm what kind of finish is needed on all surfaces and on welds, whether polished or what.
Confirm if the tank is or can be pressurized.
If this is for food products, that is kind of a specialty field, so look for someone normally doing that.