Daraja comes from Arabic دَرَجَة (daraja, “step; stairway; degree”).
Is the j silent?
no. No Swahili sound is silent
When does J sound like 'YA' and when does it sound like 'JAr?'
'J' never sounds like 'ya', it's like the 'dg' in 'bridge' Sounds are pretty consistent in Swahili, what you see is what you get