MongoDB
What is MongoDB?
From the horse's mouth: MongoDB is a document database with the scalability and flexibility that you want with the querying and indexing that you need.
MongoDB is the OG of NoSQL databases and is probably still the most widely used. It is the M in MERN. If I were to recommend databases to learn, I would recommend PostgreSQL for relational SQL and MongoDB for NoSQL.
Download
MongoDB makes it difficult to get to the free stuff. "Get Started" will take you to a registration page for their cloud offerings and there is no download link or button on their main page. You don't have to pay of course (I wouldn't recommend it for personal development if you did), but what you are looking for is their "community" edition. This is what the free version of many tools is commonly called. GitLab also has a community server that you can run yourself for free, for example.
From the main page, go to Products > Community Edition, then Download Community. From there, there are multiple options, but I recommened just clicking the latest version, Windows x64 platform and msi (installer) package, then "Download".
Install
Go to your Downloads or wherever you downloaded the .msi file, and double click to run it.