Make sure you have NodeJS installed.(NodeJS Website)

After that, navigate with a terminal to the main folder: ../Sesh.
Install all the required dependencies with npm install.

Make sure you have access to the cloud console of the project. (Firebase Project)

Navigate to the settings of the android project and download the google-services.json file. (Android Firebase Settings)

Once downloaded, place the file in the correct folder: ../Sesh/android/app/google-services.json.

Add a dev.js file in ../Sesh/src/config/ directory.
This file should contain your google access keys, the file will look like this:
(keys found at Authentication methods by opening the Google Signin tab)

export const googleClientID = 'YOUR_GOOGLECLIENTID_HERE';
export const googleClientSecret = 'YOUR_GOOGLECLIENTSECRET_HERE';

If you still have issues with the firebase configuration, follow the steps on the package's official page.
Normally these steps should already be completed, but you can try this if nothing else works.

Firstly make sure you have an Android Virtual Device (= AVD) or an actual device connected to your pc.

For an AVD you'll need to configure the android environement.
Follow the steps on this page for this: React Native Develop Environement

Select React Native CLI Quickstart, after that make sure to select Windows and Android.

For a physical device you'll need to enable usb-debugging on your phone (developer settings -> USB Debugging).
After that you can get a program to emulate your screen on your pc (NOT NEEDED; the app will also build on your phone).
Tutorial to mirror screen to your pc If you've mirrored your phone's screen you should also be able to connect with a react app.

After having an emulator, navigate in a terminal to ../Sesh and execute npx react-native run-android. This will build all the files and link the libraries. The first time, this will take a while. After first build, consequent builds get faster.

The app should be automatically installed on your phone.