How to make a tree of all subsections of a section?

Question

1C-Bitrix

Hello! Guys, I can not figure out how to do the following. There is a page, a component (catalog), on the left in the column the full tree of child sections of a certain section is displayed, for example, in the form of ul li, and on the right are the elements of the selected section. I can't figure out how to do something like this..