How to Open files and folders in same window in Sublime Text on macOS?

In Sublime Text Menu:

Preferences ->  Settings - User

Look for 'open_files_in_new_window'

And change 'true' with 'false'

Rather than editing the SublimeText default settings (as Jahnux73 suggested), you should go to Preferences -> Settings - User, and add the following:

"open_files_in_new_window": false,

The SublimeText default settings file should not be modified, as it may be overwritten by Sublime updates, and so that you'll always be able to restore the default settings. The way Sublime works is that at runtime, it reads and applies the default settings, and then reads and applies the Settings - User file. In the event of a conflict, the User settings take precedence. That way, you can add your own settings without modifying the default configs.