lichess puzzle database

How to open a csv.zst file?

You need to install the compression/decompression utility called zstandard or zstd. On linux the command to uncompress is 'zstd -d file.zst' or 'unzstd file.zst'. On windows also there is zstd, which you will have to install from somewhere. After installing it, you may just have to click on the file.

It's described on the page: database.lichess.org/#zst

Thank you. I used Peazip to decompress and then opened the file using OpenOffice Calc. It doesn't show 3,080,529 but
1,048,576 puzzles. I guess thats enough for a lifetime.

This might be helpful:

cshancock.netlify.app/post/2021-06-23-lichess-puzzles-by-eco/

You are hitting a 2^20 limit, the maximum number of rows Calc can show.
Since CSV is just a text file, you can split it into several chunks. E.g. using split from core utils or something like
github.com/BurntSushi/xsv
github.com/ribbondz/rsv