It can be very direct, with explicit language to describe the physical acts. Another way is through euphemisms, like using more flowery or veiled language. For example, instead of saying 'they had sex', it could be 'they became one in the night'. Also, it can be implied, where the author just gives hints about the sexual relationship through the characters' behavior or the atmosphere, like a charged look between two characters or a blush.
In novels, sex description can be presented in various manners. Firstly, there is the matter - of - fact way, where it's just a part of the story without much frills, just simply stating what happened. Then there is the poetic way, where the author uses beautiful language to make it more aesthetically pleasing. For example, comparing the touch of lovers to the caress of a gentle breeze. There's also the way of using dialogue, where the characters talk about their sexual experiences or desires, which can be very revealing and add a different dimension to the sex description.