Management/monitoring. It’s intended to handle multiple TrueNAS systems; if you only have one TrueNAS box, there really isn’t any use for TrueCommand IMO (which is something iX is trying to correct using TrueCommand, so they can paywall high-demand features like enclosure management).
Um, follow the docs on installing a custom app? You’ll need to know a little bit about Docker.
IMO, if you’re going to be doing much with custom apps, you’d be better off installing one of the Docker manager apps like Dockge, and then using that to run the Compose file(s) of choice: