Need help understanding the role of the db componment

Another newcomer question. I’ve gotten local and remote test environments running, but I’m still lost understanding what the role of the database really is. Does it store just usernames or ?
As I create content types, their details get stored as .js files in my project files. I’ve seen that in a local install using quickstart, your admin user is created in a sqlite db. Then you will have another db in a production environment. But what exactly do you need to synch between production and a local development instance not have environments out of synch, in terms of configuration, plugins,etc. In other words, does the admin panel store changes in the db?

System Information
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