Hello, for anyone else installing on Catalina, the answer to the 'MongoDb shuts down with code 100' error is this:

Context : (MacOS Catalina onwards)

Apple created a new Volume in Catalina for security purposes. If you’re on Catalina, you need to create the /data/db folder in System/Volumes/Data.

Use this command:

sudo mkdir -p /System/Volumes/Data/data/db

Then, use this command to give permissions:

sudo chown -R `id -un` /System/Volumes/Data/data/db

Then to start mongoDB you can use:

sudo mongod --dbpath /System/Volumes/Data/data/db