I sit in my closet!! It sounds kinda silly, and you will probably feel VERY silly just sitting in a closet, making gibberish Japanese noises into a microphone. But it's the perfect recording environment! It's small and compact so your voice won't reflect off stuff like furniture and random objects.
The way it works, in simple terms, is that you record your audio into the mic, and the room that you are in has certain "acoustics" which can change the tone of the recording. For example, the bathroom has very reverb-y acoustics, which is why u sound good singing in the shower ahaha. But the bathroom wouldn't be good for recording an utau because your voice would reverberate and echo back into the microphone, causing distortion and reflections in your samples! The closet, or at least the one I have in my room, has just plain walls, is completely boxed in, is quiet and away from the rest of the house, and I think the clothing hanging up actually absorbs the sound? don't quote me, on that ahaha. It works great if you don't have a professional studio, or that black spiky stuff that YouTubers cover their walls with (that stuff is foam, meant to absorb the sound of their voice so it doesn't echo back into the microphone)