How may I split my page in several columns?
You can split any page in columns using the page columns option in Page Properties and Page Layout.
I need to have a different number of columns in the different parts of my page. Can I achieve that with Page Columns?
No, in this case better use Sections. Add a horizontal classic Section Group with the desired number of sections (for example, 3 sections if you need to split this part of the page in 3 columns), below add another horizontal classic Section Group with the desired number of sections. In each section you may now add any type of content you wish.