YouTube takes a 1280 × 720 image of up to 2 MB, then shows it far smaller, with the length badge over the bottom-right corner. Open your thumbnail to see it at the sizes viewers actually get, checked against the upload rules.
Type timestamps in the description and YouTube splits the progress bar into named sections. The list must start at 0:00, have at least three entries in order, and every chapter must last 10 seconds or more. Edit the description below; the bar is drawn from what YouTube would read.
Every upload is compressed again for streaming. A file near YouTube's recommended bitrate gives that second encode clean material to work from. Pick what you exported to get the bitrate, the file size and an FFmpeg command that matches YouTube's published encoding settings.
ffmpeg -i input.mov -c:v libx264 -profile:v high -preset slow -b:v 8M -maxrate 12M -bufsize 16M -bf 2 -g 15 -keyint_min 15 -sc_threshold 0 -pix_fmt yuv420p -c:a aac -b:a 384k -ar 48000 -movflags +faststart youtube-1080p30.mp4Keyframe every 15 frames (half the frame rate), two B-frames and the index at the front of the file (+faststart), as YouTube lists. Exporting at 1440p instead usually gets a better-looking stream, because YouTube then also processes it in VP9 or AV1.