My kids walked themselves to school from about the age of 7 (the eldest) because I went back to work and it was just around the corner, no major roads and loads of kids walk in our area.
They have been catching public transport alone to get themselves around during the daylight hours since around 13. Not alone at night and they are currently 16 & 17.

My daughter is almost 10 & she started riding her bike to school this year. It’s 1km away from home.

Make sure they know the stop, to look out the window and know their surroundings of where to get off. If you can, ride with them a couple of times. My daughter has been catching the bus since starting high school. It took her about a week to really get the hang of it all but now she is very confident while catching the bus.

I think it depends on their maturity levels - my kids caught the school bus from prep (kindy) onwards, but that was from our front door straight to school with no stops. My kids are teenagers now and have phones - I let my 16 yo daughter get the bus on her own into the shops if she's meeting with friends but she texts me when she gets there and when she's leaving so I know where she is. My son is 13 and he gets the bus occasionally with friends but not on his own yet.