Yes. No account required. Research as many topics as you want.

Yes. LinkedIn users can follow hashtags. When you use a hashtag, your post appears in the feed of people who follow that tag. It helps people who do not already follow you discover your content.

If a hashtag has millions of followers and thousands of new posts every hour, it is probably too big for most people. Your post will disappear in minutes. Look for hashtags with thousands or tens of thousands of followers instead of millions.

Niche hashtags are specific to a smaller audience. Instead of #marketing which has millions of posts, a niche tag might be #emailmarketingtips with thousands of posts. Your content stays visible longer and reaches people who care about that specific topic.

No. Mix it up. Use different hashtags based on what each post is about. This helps you reach different audiences and avoids looking repetitive.

Most people put them at the end of the post. Some work them into the text naturally. Both work. Just do not put them at the very beginning where they distract from your hook.