Containers can’t be nested inside other containers and should really be the container for the whole page. If you need full width background but a centred fixed width section then you can use a div around the container for the background image but each block should be separate and no containers should be nested inside other .containers.
Rows are used to hold the columns once inside the container.. To nest rows they would need to be inside a column so that the column padding accommodates the negative margin on the row.
The grid has to be adhered to strictly and is why I don’t think it is suitable for beginners to use until they understand the basics of css. I have worked with bootstrap on a few client sites but I tend to avoid using the grid for all but the simplest of layouts.