my-sql-replica VS bitnami-docker-mariadb

I searched online for how to link new applications to existing dbs but the process seems to differ from application to application. For example, for firefly III and bookstack, I just had to set DB_HOST, DB_USER, etc. in the environment and I was able to connect, but when trying the exact same thing for snipe-it, the application can't find my database. This link says just set `external_links` and it should connect, but I didn't get that to work (although I think it might be because my mariadb isn't a docker application), this link tells you to map some volumes (which idk how I would do for snipeit). And there are so few results on the topic, I googled "docker link existing mariadb" and most of the other stuff I got were how to install or access mariadb, so I assume almost no one else had this question, which means either this is extremely trivial and everyone knows how to do it, or this is terrible practice and nobody bothers with this.