Requirements

Windows
.NET 8 SDK to build (running needs the .NET 8 runtime, which the SDK includes)
git on PATH when your mods folder is a git repo (mods are then packaged with git archive)
SteamCMD: required for uploading. If it is missing at start-up the app offers to download it for you (also Options > SteamCMD > Install SteamCMD)
A Steam account that owns RimWorld