Making curved objects in Hammer SDK?

I've watched a few tutorials on hammer basics but they don't mention how to make anything besides cube shapes and stretching them. How do you make other shapes or edit the shapes into forming other ones?

for my part i stuck on the skybox, the rest is "very easy" (you learn by doing)

i would try to avoid those youtube tutorials, most are clickbait/attention whores who dont know how to teach people
ie they make a series of 50 tutorials so they can get a crap ton of views to their channel

try to use the wikis and other documentation, also use facepunch as they have a mapping section with many guides

I believe the in the grid settings set the size to the highest and make the best circle you can.