Coincidentally, just this weekend I posted a video and some text on exactly this subject. Most has been said here already, but who knows it can still be of help.
https://rudysarduinoprojects.wordpress.com/2019/01/04/fun-with-arduino-03-connect-an-external-led/
https://rudysarduinoprojects.wordpress.com/2019/01/04/fun-with-arduino-03-connect-an-external-led/