A preposição "on" e "they've": Como funciona?

Olá!

I don’t know what I would do without my friends’ advice. They’ve all given me so much good advice over the years. It’s sometimes very difficult to make decisions on your own. Listening to advice can be great for helping you make the right decision. The greatest piece of advice I’ve received was from my father. He told me I can do anything in life if I try hard. He was right. I am now passing on his advice to my own children. Right now I need some advice on money. My financial advisors are telling me to be very careful. That’s very sound advice. I would advise anyone to do the same. Someone asked me the other day about the worst piece of advice I’ve ever had. I couldn’t answer. I don’t think I’ve had any really bad advice.

Porque usa-se a forma ´They’ve´? Porque usamos a palavra ´on´depois de advice?