Hi @Mat_Jung
MongoDB Atlas is the global cloud database service for modern applications.
You can deploy fully managed MongoDB across AWS, Azure, or GCP. For a simple database you only choose which provider you like, nothing more - no extra account no extra contract that is all included. In case you change your mind and want to change the underlying provider, you can do so with in Atlas (s. above the cloud database service from MongoDB. You only need to chose the new provider everything else is done for you, just wait until the migration is finished.
There is lost more just hop over to the MongoDB Altas page and read the intro and/or the blog posts.
You asked for the free tier Cluster, you will find information about this on the previous mentioned link. In short: you can start completely free with a small DB (< 0.5 GB).
Cheers
Michael