Menu issue in WordPress. How to make 2 columns?

Question

Web development

I would like to make the 2nd and 3rd menu levels in 2 columns.
Where can I look, what to read, what files to dig?
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Answer 1

Open the file where the menu code is written (header.php, content-header.php or something like that).
There the menu is most likely through a list or table.
If through a list - make a table and instead of writing each item in a new row - write every second in the same row, but in a new one

Answer 2

Try css columns, or you can write your own walker for the menu, which will display the markup you need.