This is part of a full-stack challenge.
Let's start by trying to make all layers working by themselves: React Native, NodeJS + KoaJS and MongoDB.
In this tutorial we'll be setting up MongoDB following these instructions on Ubuntu 18.04

MongoDB

Import the MongoDB public GPG Key:
wget -qO - https://www.mongodb.org/static/pgp/server-4.2.asc | sudo apt-key add -

Create a list file:
echo "deb [ arch=amd64 ] https://repo.mongodb.org/apt/ubuntu bionic/mongodb-org/4.2 multiverse" | sudo tee /etc/apt/sources.list.d/mongodb-org-4.2.list

Reload the local package database
sudo apt update

Install the latest stable version
sudo apt-get install -y mongodb-org

Start MongoDB
sudo service mongod start

Check log
cat /var/log/mongodb.log

Stop and Restart commands
sudo service mongod stop
sudo service mongod restart

Run mongo shell to connect to a MongoDB instance on default port 27017:
mongo or mongo --port 27017

Display the database you are using
db (returns the default database 'test')

Insert one object in the collection
db.collection.insertOne({ title: "Stampler" })

Query/Select it
db.collection.find({}) or
db.collection.find({ title: "Stampler"})

That's it. The next step is to make so NodeJS can query objects in our MongoDB instance. But before we'll set up React Native and NodeJS + KoaJS.
Feel free to comment below or hit me on twitter @HeyMarkKop