Many YouTubers are able to upload content daily because that's all they do. Many of them are "full time YouTubers". They have enough time on their hand to produce and upload contents daily.
They are also YouTubers that produce many short clips at a time, they shoot a lot of videos in a single day and they upload this videos on a daily basis. For instance, a YouTuber can make more than one make up video a day when she has enough time and she already has content to upload every day even when she has a very busy day.
Yes, many you tubers do upload video content daily. How? Probably that's their life. Totally. You can buy content and upload it to save yourself the trouble but it won't be unique, and thousands of others will be doing the same. No-one will come back to see your videos on the following day if you've presented content that they've already seen elsewhere or their friends have.
Viewers want to be able to say 'Look what I've seen" and be the first, the only, to be able to say that. To upload videos daily you need to have a constant stream of content. For instance, if you're touring Vietnam, there's plenty of chance to video short clips throughout the day, enough to upload one daily. whether you'll also have time to enjoy Vietnam, that's another matter.