To install MongoDB on Ubuntu, follow these steps:

1. **Import the MongoDB public GPG Key**
   MongoDB is signed with a public GPG key to ensure the authenticity of the software. Import this key with the following command:

   ```bash
   wget -qO - https://www.mongodb.org/static/pgp/server-4.4.asc | sudo apt-key add -
   ```

   Replace `4.4` with your desired MongoDB version if needed [2].

2. **Create a List File for MongoDB**
   Create the `/etc/apt/sources.list.d/mongodb-org-4.4.list` list file using the echo command:

   ```bash
   echo "deb [ arch=amd64,arm64 ] https://repo.mongodb.org/apt/ubuntu focal/mongodb-org/4.4 multiverse" | sudo tee /etc/apt/sources.list.d/mongodb-org-4.4.list
   ```

   Again, replace `4.4` with your desired MongoDB version if needed [2].

3. **Reload Local Package Database**
   Reload the local package database to include the MongoDB packages:

   ```bash
   sudo apt-get update
   ```

4. **Install MongoDB Packages**
   Install the MongoDB packages:

   ```bash
   sudo apt-get install -y mongodb-org
   ```

   This command installs the latest stable version of MongoDB [0][2].

5. **Start MongoDB**
   Start the MongoDB service:

   ```bash
   sudo systemctl start mongod
   ```

   Enable MongoDB to start on boot:

   ```bash
   sudo systemctl enable mongod
   ```

   Check the status of the MongoDB service:

   ```bash
   sudo systemctl status mongod
   ```

   This should indicate that the service is up and running [2].

If you want to prevent unintended upgrades of MongoDB, you can hold the packages at the current version:

```bash
echo "mongodb-org hold" | sudo dpkg --set-selections
echo "mongodb-org-server hold" | sudo dpkg --set-selections
echo "mongodb-org-shell hold" | sudo dpkg --set-selections
echo "mongodb-org-mongos hold" | sudo dpkg --set-selections
echo "mongodb-org-tools hold" | sudo dpkg --set-selections
```

This ensures that even when a newer version of MongoDB is available in the repository, it won't be upgraded unless you manually choose to do so [0].