You can build the app from source and distribute it within your team or company either using the App Stores, Enterprise App Stores or EMM providers, or another way of your choosing.
At Mattermost, we build and deploy the Apps using a CI pipeline. The pipeline has different jobs and steps that run on specific contexts based on what we want to accomplish. You can check it out here.
As an alternative we’ve also created a set of scripts to help automate build tasks. Learn more about the scripts by reviewing the package.json file.
When building your own Mattermost mobile app, you will also need to host the Mattermost Push Notification Service in order to receive push notifications.
See Setup Push Notifications for more details.