For your first question:
.Params.tags will return a slice of strings (array), it’s a list of tags associated with the page.
So you could use the in function, (if in .Params.tags "link")
For your second question, this post should give you more context on the dot etc…: https://regisphilibert.com/blog/2018/02/hugo-the-scope-the-context-and-the-dot/