Is it possible for new industries to be built as time goes on, or do the industries which are present at the start of the game remain static for the duration?
New industries are build if city grows.
Present industries stay more or less constant (maybe new industrie connections).
So is it not possible to get new oil fields, etc., since these aren't located in cities?
If you expand the map during a game (using the map editing tools as the Public Service player), how do you populate the new portions of the map with industries randomly (as opposed to putting them in yourself)?
Industry CHAINS are built somewhere, triggered by a city p****ing a certain number of inhabitants (in default setting for pak64 2000, 4000, 8000, 16000, ... )
Chains start from an end consumers, which are usually in cities. (The only exception in pak64 is the garbage incinerator.) If an end consumer consumers more than one product, only a production chain for this product is built. Next trigger event the chain for the second is built etc. until all slots are filled and the next end producer is built.
You can try this with the increase city population tool for the public player.
You could place there by your own new industries or you wait until ST places there some industries by city growing or you use the "Increase Industry density" toll as public player.