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