I'll try to give a bit of an overview to help answer the first three questions.
In Notion, all your documents are basically a big nested hierarchy of pages. Pages (generally) exist within another page, rather than simply being linked to (though you can link to pages in a different part of the hierarchy through either their URLs or by aliasing the page, letting a document live in more than one part of the tree at once). By nesting the pages, you get a nice table of contents in the sidebar.
Pages are composed of "blocks". These are just page elements, and most of these are the kinds of things you'd expect to see in a document (paragraphs, images, sections, bullet lists, etc.). There are some special ones, like TeX equations, inline discussion forums, and other things, but these are entirely optional.
Unlike a document in Word/Google Docs, blocks are more of a real entity that you'd interact with. For example, you can select a whole block (and any children) at once, drag-and-drop them elsewhere in the document (even to the left or right of other blocks, creating columns), link to them, and even transform them into other types of blocks (like bullet lists to numbered lists).
Pages themselves are actually a kind of block, which is neat. Let's say you have a giant page full of content, and you want to split out some of the sections in the page and put them in their own page. Simply click the block options for that section and choose "Turn Into -> Page", and now it's a new page linked where the section used to be, at the right place in the hierarchy.
Task Boards, Design Specs, etc. are really just templates. They're nothing special. You can make them by hand if you want to, or change them however you want. They're really just there to help you get going if you have use of them. No different than templates in Work, Excel, etc.
As far as WYSIWYG goes, since you're dealing with these page elements (bullet points, section headers, etc.), it really is pretty much WYSIWYG. You can select text or a header or whatever and change the styling a bit (colors, sizes, fonts, all to a limited degree).
Along with that, though, there's a form of Markdown processing. These are really more like keyboard shortcuts, though. Type "# " to get a section. Enclose something in backticks to make it appear as code. "* " for a bullet list. Things like that. It's not a full Markdown engine, though. There are a few bugs with some of these shortcuts, but they mostly work.
Editing in Notion is definitely a bit different, because you're kinda forced to think about blocks and hierarchy a bit more, but they've improved much of this over the past couple of months to make it more natural to work with. They've been hard at work with improvements, from the looks of it, so I'm expecting a lot of this to really improve more going forward.