How to create classes for text blocks or divs in sidebars

Steps:
Create a page, full-width. Using layout builder, add a 1/3 and 2/3 div to the page.
Add a form to the 2/3 div.
Create a selection of text boxes in the 1/3 div to provide instructions for the form.

Problem:
1) It seems that I cannot nest divs inside the 1/3 div, so I am obliged to use text content boxes.
2) Using one single text box does not allow me to space the text adequately (for example after an introductory block, the next block needs to be 100px down the page so that it sits in the appropriate place). I cannot add spacer items to the text block.
3) If I create individual text blocks and add spacers between them, they are only appropriate at certain screen resolutions and at mobile or tablet resolution are hugely inappropriate.
4) It doesn’t appear to be possible to add classes to the text blocks, nor to individually control them in any way.

Question:
How do I add classes, to specific divs or to the text blocks inside them? With plain html it is childs’ play, but I can see no possible way to attribute classes to these components and therefore to control them individually at varying screen resolutions.
I thought maybe it would possible to nest divs inside divs and attribute appropriate classes to them, but the layout builder doesn’t seem to allow that. In addition, there isn’t a ‘text’ view that would allow me to manually edit the layout.

My aims are to ensure text blocks/components within a page are formatted correctly, with text located appropriately. At lower than full-screen res, I would need to edit the component sizes and even change visibility f some items to none. I’m sure it mst be easy, but I just can’t see how to add the classes to allow all this.

Thanks