Have a look at Blender's add-ons folder for many examples. C:\Program Files\Blender Foundation\Blender 2.91\2.91\scripts\addons if you are on Windows.

Let's have a look at MeasureIt add-on for example. It has multiple files:

If you open the __init__.py, you can see that measureit_main is imported

and if you open it, you can see other imports:

You could also have a look at How to run a python script external to Blender to run bpy commands in Blender'spython console

Have a look at Blender's add-ons folder for many examples. C:\Program Files\Blender Foundation\Blender 2.91\2.91\scripts\addons if you are on Windows.

Let's have a look at MeasureIt add-on for example. It has multiple files:

If you open the __init__.py, you can see that measureit_main is imported

and if you open it, you can see other imports:

.zip files that you use to install the add-ons are just simple zip files containing a folder named after the add-on and all the files in it.

You could also have a look at How to run a python script external to Blender to run bpy commands in Blender'spython console