This completely depends on the individual YouTuber and the type of content they are making. A gamer will record using a webcam and screen recorder, while a vlogger might use a webcam or mobile phone to record. DSLR cameras are often used as well, although these days mobile phones record just as well! Travel or Action vloggers use an action camera such as GoPro to record in bumpy or wet conditions.
Whatever you use to record, make sure the resolution is somewhere between 1080 and 4K. Videos in that resolution will look high-quality, yet won’t exceed YouTube’s size limit.