Buying the game and expansions

Hello! Just a question about getting the expansions on steam since theyre on sale. I bought the game in physical format when it first came out, now I dont know where the cd is and I also have ambitions expansion on cd. How does it work for me to buy some expansions on steam? Do they carry to my origin account? Do I need to buy the base game even if I have already bought the physical version? If anyone could enlighten me on these questions Id be very grateful! Thank you for your time!