Search for tModLoader directly on the Steam store. It is available as a free, standalone application for anyone who owns the base game. Click install, and it will configure itself alongside your native Terraria installation.
Linux users searching for "Terraria Linux download free" have more legitimate options than they might expect. The game officially supports Linux through Steam, runs beautifully through Proton when needed, and can be purchased DRM-free from GOG. The tModLoader modding platform is entirely free and open-source, offering thousands of hours of additional content for those who already own the game. Open-source clones like TerrariaGL and the Open Terraria Project provide completely free Terraria-like experiences, though with less polish and depth. terraria linux download free