I am new to the photography, and I use a Nikon D3400 and 18-55 VR kit lens for casual photography and once in a while I shoot video in 1080 60p. But everywhere in youtube I see people saying that Canon/Sony is good for video. What advantages do Canon/Sony have? I have read that contrast based autofocus is more accurate than dual pixel phase detect autofocus.So Nikon should be better and in pro work people use manual focus anyways. Please explain....
WIth Canon it is mainly the autofocus. Seriously.
Those who have shot with all of the brands, because they are 'professional' or 'semi-professional' reviewers, or they are actual working pros that rent whatever they think will work best for a particular job and have used all of them at some point, know that Canon's Dual Pixel CMOS AF just works for video. Nikon's video AF doesn't work near as well.
It's nothing to do with the difference in accuracy between off-sensor based PDAF and on-sensor based CDAF, because Dual Pixel CMOS AF is a hybrid contrast and phase difference AF system that is located on the actual imaging sensor, and thus is not subject to the same calibration issues that separate PDAF sensors are subject to.
But beyond all of that, it's mostly about the way those hardware assets are used by the firmware running them. Dual Pixel CMOS AF is the first AF system in a DSLR that can continuously AF while recording as well as, or even better than, dedicated video cameras have been doing for years.
... in pro work people use manual focus anyways.
Well, they often do, but not always.
In the past it was likely because no AF system available could do as well as skillful manual focusing. And while many seasoned pros still use manual focusing much of the time for certain types of footage, not everyone shooting video with a DSLR or MILC is as skillful with manual focus as those seasoned pros are.
In more current times, pros still use manual focus for some types of work, but they are also happy to use AF if the AF system is good enough to give them what they want/need. Until Canon introduced Dual Pixel CMOS AF, none of the AF systems available for shooting video with a stills based ILC was "good enough" for many of them. Now, for some types of shooting, it is good enough.
The autofocus is not the only thing people are looking for when looking for a camera for video. Nikon had long time problems with the autofocus while making a video which just got better with the d850 and the z7.
Canon has a very good autofocus system with its dual pixel autofocus and also a lot of people really like the color directly out of the camera because it gives quite good skin colors.
Sony is also good with the autofocus and has ok color science.
The biggest problem with nikon video was the codec because nikon introduced log with the z7 where canon and sony had this a longer time and with log you get a lot more dynamic range.
But Also this topic is not Photo related (aside from that the cameras can do photos as well) and should be asked at video.stackexchange.com