Hi Mehtaz,
Thank you for showing me this page!
This URL is based on version 3, I use version 4.
In version 4 the interface is different, but this URL showed me that the option for inner row/section is already for years in sppagebuilder :)
What I tried last week in version 4 was probably the wrong way.
In version 4 you can add a row via 'addons' > row, a new row with two columns is added to the section.
From there you can add/drag extra columns to the (inner) row via 'addons' > column.
So...in version 4 it is not called inner row or nested row but just 'row'.
Thank you for pointing me in the right direction!