TikTok has no banner upload. What people call a TikTok banner is a trick: three videos pinned to the top of a profile, each with a cover image that is one third of a wider picture. Seen in the grid, the three covers read as one banner across the top.
That makes the shape of a profile tile the thing to design around. TikTok video covers are prepared at the full 1080 × 1920 px frame of the video, and the profile grid shows a centre crop of that cover at 3:4. So the part of your banner that appears in the grid is the middle of each cover, not the whole thing.
This preset cuts three panels at the profile tile shape, so what you see in the preview is what shows in the grid. When you set each panel as a cover in the app, you are placing it inside the taller video frame, and TikTok crops back to the tile shape from the centre. Keeping the artwork centred is what makes the two crops agree.
Two more things TikTok does to a cover. The first line of your caption is drawn over the bottom of it in the grid, so keep the lower part of each panel clear of anything you need read. And the tile only stays in place while the three videos are pinned, so pin them and leave them pinned.
Nothing is uploaded here.