I know of two ways features can be developed.
- The Tutor v1 plugins
You can think of the Tutor plugins are completely separate apps that will connect to Tutor and Open edX platform via hooks. For example, an app that collects analytics data Figures or an app that is an alternative to LMS Richie.
The official documentation for creating a Tutor plugin is here: Creating a Tutor plugin — Tutor documentation
The official template: GitHub - overhangio/cookiecutter-tutor-plugin: Cookiecutter for tutor plugins
You can take a look at the official plugins for a starting point: Overhang.IO | Tutor Plugins
- xBlocks
I am not familiar with xBlock development, but it is easy to add custom xBlocks to Tutor: Open edX development — Tutor documentation
Open Stack tutorial: XBlock Development - OpenCraft Technical Documentation
xBlock template: GitHub - openedx/edx-cookiecutters: Open edx public templates for apps, libraries and services.
There is also the third way of custom forks, however, it requires more development time. You can check my tutorial on starting with custom forks: FAQ: Running an edx-platform Fork with Tutor