Sorry for the delay in answering.
2. Yes, when the Single product builder is enabled you can’t use default layouts and need to create the page layout from scratch, for example – https://prnt.sc/udjjdj https://xstore.8theme.com/elementor/demos/niche-market/product/wooden-chair/
At this moment we don’t have video tutorials for the single product builder. We plan to create it in the future. Single product builder is similar to header builder that we have but with other elements and to manage the columns you need to use sections and change their width from 0 to 100% to create layout you need. Every new section displays next to the previous one if the width of the column allows or goes to the next row. Every element has a number of additional settings to change its styles, like colors, spaces, paddings etc. You can check the every element (tabs for example) and study.
Of course, single product builder and its settings do not include all the possible options and maybe you will need some custom CSS to change something. In this case, you may try Yellow Pencil plugin. It helps to edit styles in case you don’t have CSS skills.
3. Portfolio project page can be edited using page editor, so you can edit it like pages – https://prnt.sc/udjm0c
Regards