Can someone explain to me how you are able to both sell to open and buy to open a put option and call option

I’ve been doing a lot of research and studying of how options work before I start doing them and one thing I don’t really get is when you go on to a lot of online brokerages to open a contract, such as Robinhood, there is a buy and sell for call and put options. I thought you bought to open for call options and sold to open for put options. I know I’m missing something, but how are you able to do the inverse for each?