short answer: you can run most of the Python API without opening the Sim4Life user interface.
you have different options
you can call Sim4Life.exe --run your_script.py
you can write scripts and run them using the python.exe in the Sim4Life installation
you can
create a virtual environment with the Sim4Life packages, e.g. "C:\Program Files\Sim4Life_6.2.2.6592\Python\python.exe" -m venv .venv --system-site-packages and then use the python from this venv to run your script
The first option opens Sim4Life, runs the script, and closes Sim4Life again. It probably is the easiest option to work robustly.
The latter two options run without the UI and by default without an application. In most cases, you will need an application though (e.g. the Application initializes the active model, which is needed for modeling). For Python scripting without the UI you can create a console application. We do this e.g., to run Python tests without opening the UI (which is slower). To create a console application you can usually call:
import XCore
XCore.GetOrCreateConsoleApp()
# now you can do some stuff
import s4l_v1 as s4l
sphere = s4l.model.CreateSolidSphere(s4l.model.Vec3(0.0), 1.0)