Difference b/w Spider vs Crawler vs Robots?

I am running a ecommerce website and there is sign-in page like this (amazon.in/-/hi/ap/signin?). So, I am confusing can i index this page or not? Is it good for SEO when i index this page or is it good for SEO when i de-index this page?

Tell me the Answer with proper reasons.

Recently, I have created a new sub-directory abc.com/au and I have implemented the GA TAGs, and Search Console Tags properly. But, also my directory sessions are viewed on my old Analytics. Then, suggest to me how can i solve this problem?

Recently I heard the news about FAQ Schema is no long. But on my website I have implement the FAQ Schema. SO, can you suggest me I can remove the code from that page or leave the same. Is it FAQ Schema code harm the website speed or not?