I asked this same question a couple weeks ago in a different forum. This is the link to that question, and posts 3 and 5 give a good description and a link to an article on the topic. It’s not a long thread and worth reading (only 5 posts).
The TLDR version is that the value of big golf courses, partial developments, and sky scrapers are more complicated than your typical single family residential bungalow. There are actual appraisers involved but they can be influenced and pressured to change their evaluations by arguing about various real influences on properties values.
It’s apparently common to argue to the property tax assessor that a piece of undeveloped swampland in the floodplain with no current development permits is worth very little, while arguing to the bank that this same pristine beach-front property is worth millions as soon as approval to build luxury 200 homes comes through in a week.
The question is did Trump take this tactic far enough often enough to be considered criminal?