A question about Blogger?

Once you have a dominion name, you can create sub domains under it without have to buy another dominion name.  Or you could just put each blog on a different page under the main domain.

If you allow your domain registration to expire, ICANN will shut down access to it and no one will be able to reach your site.  The name would go back up for sale and someone else could buy your name and use it instead.  They would not be able to reach your actual site, but they could create something that enough like it that they trick you users into using their site instead.

you can always create subdomains and tie them to the blogs. like "sub1.domain.com", "sub2.domain.com", etc.

Normally you buy a domain "name.xxx" you can add subdomains to it such as "sub.name.xxx". Every domain would be a separate but you can usually add subdomains free. You must renew it every year.