I need to read a SERVER_AND_PORT from the custom.property file. let me know how

You can create a custom function that returns the following:

ConfigurationLoader.getConfiguration().getServerAndPort();

I am using
=extract(linktodocument(0, "doc"), """", "doc/")
to get the currently configured base url.

I keep them in a constants and leverage a rule to build URLs.

What if I need to see whether the given environment is production or development? As I need to create some different folder based on the environments.

Any of these solutions will apply to this other requirement. Usually DEV environments have a different URL such as "myappiandev.appiancloud.com" in that case you can use any of the suggested approaches (mine, Jorge's, Moritz's) and parse the returned value in a gateway (e.g. XOR) and configure it with a condition of type: if the URL matches/contains DEV then take this path in the model, otherwise take another path

To parse the URL you can use the find() function forum.appian.com/.../Text_Functions

in our case, the url is not specifing whether the url is related to development or production