In industrially produced soup, how does each can contain equal parts of all ingredients?
Obviously, every manufacturer is going to have their own proprietary methods.

However, canned goods are often made by combining ingredients (possibly partially cooked) directly into the cans, and then pressure cooking them in the can as part of the canning process.

So, for example, the broth, some celery, and some carrots might be added to the open can in measured amounts.  Then, the can is sealed, and pressure cooked to both sterilize the contents, and to cook the celery and carrots through.