First I suggest reading Building from Source for the initial setup and it introduces you to the launch scripts a bit more.
Again, if you're using vscode/vscodium just press F5. To select the right launch profile either use the side panel or press CTRL + P, type debug, then a space, and select the launch profile from there.
The Phobos and Lua prefixes are currently relevant since phobos itself is still written in .lua files. That means the Lua prefixed launch profiles will run directly from source while Phobos will use phobos itself to compile src and then run and debug out/debug. This, however, uses the Build Debug task, which uses src to compile itself, which means if there is a bug then you can't use that to actually debug it. This will change once I require you to have a previous version of Phobos installed to compile Phobos.
Run one of these, whichever is the most applicable:
# run directly from the source tree
launch_scripts/phobos_dev linux src src/main.lua -h
launch_scripts/phobos_dev osx src src/main.lua -h
launch_scripts/phobos_dev windows src src/main.lua -h
# run generated output
launch_scripts/phobos_dev linux out/debug out/debug/main.lua -h
launch_scripts/phobos_dev osx out/debug out/debug/main.lua -h
launch_scripts/phobos_dev windows out/debug out/debug/main.lua -h
# same with release
# run generated output directly (can only run the main.lua file with this approach)
out/debug/phobos
# or
out/release/phobos