What your video really is
Your computer tells you the duration and the dimensions, and that is it. But before it goes on a timeline you need more: the true frames per second, whether the file is variable frame rate (the phone-video curse) and whether it carries an instruction to turn it. The video never leaves your browser, and it is not even read in full.
23.976 is not 24, and the audio that drifts
Frame rates born from cinema are not round numbers: 23.976 fps and 29.97 fps are really 24000/1001 and 30000/1001, a leftover from the switch to colour television. It sounds like nitpicking and instead it is the commonest cause of audio drifting: round it to 24 or 30, or drop a 29.97 clip into a 30 project, and you get one frame out every thousand, which is a second every half hour. Here the number is worked out from the timings written in the file, not rounded.
Variable frame rate: the phone-video curse
A phone filming in the dark lowers the frame rate to let more light in, and a screen recording skips frames when nothing changes: that is variable frame rate (VFR). The file is perfect to watch, but a timeline wants a fixed step, and when you import it the audio starts sliding away little by little: by the end of the edit it is half a second out and you cannot see why. Here you are told beforehand, and how many frames are off the beat.
The video that «comes out sideways»
When you turn your phone, the pixels stay written the way the sensor took them and the file gets an instruction to rotate. The phone carries it out, so does your computer's player, but some editing programs do not: that is why the same file looks upright in one place and lying down in another. Here the rotation written in the metadata is read and stated for what it is, along with the real pixel dimensions.
The file is not uploaded, and not even read in full
An MP4 (and a MOV, which is the same container under another name) is made of boxes, and the one that matters is the index: inside it is the duration of every single frame. The index weighs a few dozen kilobytes even for a two-hour film, so only that is read here: a four-gigabyte file stays on your disk, and the answer comes instantly. With other formats (WebM, MKV, AVI) the index is built another way: there you get what your browser's player can say, without the rest being made up.
What it does not do
It changes nothing and converts nothing: it only reads. If the file is variable frame rate, or in a format your program will not digest, converting it to a fixed step is done with Convert video to MP4; to make it lighter there is Compress video, and to know whether the loudness is right for social media there is Video loudness (LUFS).