[Manal]

Can I create a blog in different domains and link to my site? Is this a good backlink?
Different sites offer free blogs at the subdomain i mean

That is called as PBN. You should try not to practice such things since Google restricts PBN(I heard it somewhere). If you really want to do this, you should maintain those "other" domain names frequently with quality post and run along with them with all. It's time-consuming but if you put proper efforts into it, it would surely work.

Just registering domains from freenom, hosting it somewhere and putting

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<a href="abcdlol.tld">blablabla</a>

would not get you anything.