Most of the new territories is not highly developed. Kowloon and hk island are both entirely urban.

Yes. https://en.wikipedia.org/wiki/Hong_Kong#Geography, https://en.wikipedia.org/wiki/New_towns_of_Hong_Kong.

You can be smack in the middle of Central, and then walk up a path behind the Cathedral of the Immaculate Conception and be on Old Peak Road hiking up to Victoria Peak. Or be in Wan Chai, take the stairs next to the Rolls Royce car dealer, and be on Bowen Road Fitness Trail and in Aberdeen Park within minutes.

HK island, New Territories, the outer islands and even large parts of Kowloon are predominantly green, parks and forests.

As GP mentioned, occasionally the subway operator MTR will build a new station, build a shopping mall on top, and housing for 50,000 people.