I want to keep emacs open most of the time, and then whenever I open a text file, python file, etc (from nautilus/finder), I want it to open as a new buffer in my current emacs
You want emacs client.
Basically, set emacsclient as your default editor, and add (server-start) somewere in your emacs config.
There needs to be a running Emacs instance for emacsclient to work, but if it's a hassle it's possible to have a headless Emacs launched at login.
Just for your information, If you are using GUI version on Mac, the dir os emacsclient is below:
/Applications/Emacs.app/Contents/MacOS/bin/emacsclient
Did you start server in emacs? Make sure that you add (server-start) to your .emacs or do M-x server-start from an existing EMACS session.
Conceptually, it's really easy, just do emacsclient file-name on the command line or where you name the program.