Build one Unreal Engine Plugin only

Example building Unreal Midi on my machine:

G:\5.0.3-release\Engine\Build\BatchFiles>RunUAT.bat BuildPlugin -plugin="G:\5.0.3-release\Engine\Plugins\Marketplace\MidiAsset\MidiAsset.uplugin" -package="F:\temp\UnrealMidi" -TargetPlatforms=Win64

Building voicechat on other unreal engine

C:\UnrealEngine\Engine\Build\BatchFiles>RunUAT.bat BuildPlugin -plugin="C:\UnrealEngine\Engine\Plugins\Marketplace\UniversalVoiceChatPro\UniversalVoiceChatPro.uplugin" -package="C:\temp\VoicePluginInstall" -TargetPlatforms=Win64 -VS2019

Build spline system for editor:

Add sources to Plugins\Marketplace, generate project files for unreal, recompile unreal, launch editor, enable plugin

Reference: https://dev.epicgames.com/community/learning/tutorials/0RG4/compiling-a-plugin-for-a-new-engine-version