Requirements:

• Google Chrome installed
• Get cookies.txt extension installed
• youtube-dl installed
• FFmpeg installed
• Active bilibili membership (to get 4K streaming perk)
• Basic JSON and Bash knowledge

Steps:

• Open the video page you want to download with Google Chrome
• Open Web Inspector and refresh the page
• Change video quality to some lower ones (like 480p) and change back to 4K
• Search for player/playurl in Inspector
• You should get a JSON payload returned:

Then select one of the most recent requests with accept_quality that has 120 in array:

Then select down to the data.dash.video[0].baseUrl and data.dash.audio[0].baseUrl. Remember these URLs:

Open Get cookies.txt extension and download your current cookies.

Download video and audio with youtube-dl:

youtube-dl --cookies bilibili.com_cookies.txt --referer 'https://www.bilibili.com/video/BV1d5411g7g2' 'https://xxx.mcdn.bilivideo.cn:4483/upgcxcode/12/67/336676712/336676712-1-30232.m4s...'

Then you will get two files like the following:

• 20210510 336676712-1-30280 [336676712-1-30280].m4s
• 20210510 336676712-1-30120 [336676712-1-30120].m4s

Rename the video extension to mp4 and audio to m4a. Play them to check if everything works.

Combine these two with FFmpeg:

ffmpeg -i 20210510\ 336676712-1-30120\ \[336676712-1-30120\].mp4 -i 20210510\ 336676712-1-30280\ \[336676712-1-30280\].m4a -c copy output-combined.mp4