Running TShock, Not Vanilla Terraria
Spawnly's Terraria servers run on TShock, a widely-used server mod that layers admin tools, permissions, and remote management on top of the base game. TShock is installed as a standalone binary rather than through SteamCMD, so you won't see Steam-style branch options anywhere in the panel.
Choosing Your Server Software
The Software tab shows the TShock build your server is running. Terraria only ships a single Public branch, so the tab is mainly there to trigger an update to the latest build. Saving a change here restarts your server.
Next Steps
See Installing TShock Plugins on Terraria for how to add plugins via the ServerPlugins folder. Take a backup before adding new plugins so you can restore your world if one conflicts with TShock.