1) depends. Specifications should state bowl dimensions or overall dimensions

2) depends on the product chosen. Engineered floors could be anywhere from 1/3” -3/4, solid hardwood are usually 3/4”. Underpayment depends on the product and installation.

Bowls are measured by exterior sizes. The internal size should be listed as well. But a 30" bowl is the exterior dimension, as those flanges have to be accounted for when fitting into the sink base cabinet.

Plywood, unless it is the expensive high quality $100 a sheet stuff, is not suitable for a finished floor. Despite all of the vloggers online. . Plywood is much softer than a true engineered wood floor, and will have internal voids that can collapse and cause large dents on the surface when weight is placed on it. It is a very temporary solution that is not all that cheap.