Learning Music Notation
Self-guided music theory and notation tutorial:
https://www.musictheory.net/lessons
A similar free resource that also has exercises for sight reading and ear training:
https://www.teoria.com/
Sight-Singing Practice
Here is a video with hours of sight reading practice, slowly increasing in difficulty:
https://www.youtube.com/watch?v=enHCvaF7mas
Here are some good videos for practicing sight singing and holding your part in an
SATB score:
https://www.youtube.com/watch?v=RHznYSihf4s
https://www.youtube.com/watch?v=ieXl5erp4yE
https://www.youtube.com/watch?v=axe_LjTK9XE
A more advanced 5-part sight singing practice:
https://www.youtube.com/watch?v=-uiG_T2gOFY
Pitch Accuracy
A great video for getting your ear more sensitive to very small changes in pitch:
https://www.youtube.com/watch?v=IyMS1pCbaJI
Game for identifying small changes in pitch:
https://checkyourpitch.com/listen
Game for matching pitch with visual monitor (you can sing it in a different octave):
https://checkyourpitch.com/sing
Here is a big collection of songs to use to sing and recognize any interval:
https://www.earmaster.com/products/free-tools/interval-song-chart-generator.html