The key boundary is further preparation beyond freezing or simple steam/boil cooking. A frozen vegetable that has been seasoned, sauced, breaded, or mixed with non-vegetable ingredients in a way that constitutes a prepared food is generally classified in Chapter 20 rather than here.

Yes, at the subheading level. Single-vegetable frozen products are classified by vegetable type (e.g., 071021 for peas, 071030 for spinach), while mixed frozen vegetables — regardless of which vegetables are included — are consolidated under 071090.