My second cake! How can I get smoother buttercream for frosting?

Hello everyone! This is my second cake ever that I made for my sister. It has American coffee buttercream with a chocolate drip! I am having a hard time understanding how soft buttercream should be for frosting the cake. Everytime I make it, it feels too stiff to smooth like in the videos I've seen. I wanted the sides smooth but had to use the cake comb pattern to make it look better because it started crusting too fast and I was too slow frosting it. Should I keep adding heavy whipping cream until it's soft, or should I try swiss meringue instead? Any feedback would help! Thanks!