Cross posting is re-posting a post from one community to another. Self cross posting is doing that to one's own posts. I don't like it so don't allow it in my communities.
People do it as they think their post gets more exposure. I mute them though, so they don't.
Before I publish my post, I click on Advanced settings, and I see an option to add the post to my own blog. Is that considered " self cross post "?
Nah, I add all my posts to my own blog. That's totally fine.
The difference between BLOG and POSTS is that your post-feed is just your posts. Your blog-feed is yours (if you put them there and any your repost (which is different to cross post.)
Below circled: Cross post
Below arrow: Reblog/repost
The latter will put that post onto your BLOG-feed and anyone who follows you will see it. I rarely do either preferring to keep y feeds focused on my own work. Instead I vote a lot.