Is copywriting the same as ad writing?

Do all copywriters have the ability to write highly converting sales ads
or most of them focus only on writing interesting content for a blog
without a sales pitch in it?

NO.

Do all writers have the ability to write highly converting horrors?

If you're looking for ad copywriters, then yes, they should be able to write converting ads.

However, I could call myself a "copywriter" and sell my services without ever having written a converting ad (although, if I can convince you to buy my services off an ad I wrote, at least I'm starting somewhere).

When you buy any service, do your due diligence, ask for previous samples, and select your providers carefully.

Dont ask for samples, ads writing is serious mon(k)ey business so ASK for statistics. How succesful conversion rates his ads had. Opinions doesnt care, just figures counts.