Parasite or not for authority site

Parasite or not for authority site
I am going to be building an authority site, i dont know what i should use, lets say the niche would be fitness, is it better to use yourfitness.com (example) or fitness.xxxx.com (xxxx.com being the domain that gets me 79DR on ahrefs instantly) also the parasite wont get deleted because its not a medium or tumblr but instead a real domain so i can have it forever.
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What are your thoughts ?
You're contradicting yourself in the question you asked.

If you're going to build an 'authority' site then by default it'll be on a domain that you own.

Remember, parasites exploit the authority of Web 2.0 sites (which allow two way interaction), if your goal is to build an authority website and if you're successful (which requires a lot of effort and money), in the future, people might use your site as a parasite.

On the other hand, it would be a good idea to claim the name on all the social profiles. (Hint: https://knowem.com)
I..... (xxxx.com being the domain that gets me 79DR on ahrefs instantly).....
google does not care about ahref scores. from google perspective subdomain.domain.com and www.domain.com are 2 different websites.

you are 100% better off building out your own .com domain. you will have a lot more control, specially when it comes to on-site optimization.

even if your xxxx.com that you mentioned above is not web 2.0, you are still better off with building your own domain. make sure you get all social media accounts for it when you register it. use https://www.purewhois.com/ to look up domain names and not godaddy or register.com, since these sites have a tendency of registering searched domains so that they can turn around and sell it to you at a much higher price.
I am going to be building an authority site... the domain that gets me 79DR .
You should understand, that the only authority that you will receive in this way will be only in Ahrefs.
So, if you expect traffic from Ahrefs to your site - you will get what you try to build.
And please remember that Google use Pagerank at url level instead of stupid metric at domain level, such DA/DR
They repeated it many times.

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You should understand, that the only authority that you will receive in this way will be only in Ahrefs.
So, if you expect traffic from Ahrefs to your site - you will get what you try to build.
And please remember that Google use Pagerank at url level instead of stupid metric at domain level, such DA/DR
They repeated it many times.
Google staff have also stated that metrics measured at the domain or site level are relevant for establishing authority:

“…getting a link from a high PageRank page used to always be valuable, today it’s more the relevance of the site’s theme in regards to yours, relevance is the new PageRank.”

Ahrefs and MOZ also have url level metrics (UR and PA). Google may have the most information, such that its metric is as fine as a 4k TV image, but that doesn't mean that the 720 resolution picture provided to us by DR or DA are worthless or 'stupid.' Since G won't share its PageRank data publicly anymore, most of us are content that there are third party means of giving us a current, approximate sense of authority for linking and rank building purposes.
The site relevance it's called topic distance, aka Majestic TTF
The higher distance between the topics - the less Pagerank is passed via link.

The lower trust - the less Pagerank is passed via link.
Where trust in NEW Google Pagerank patent it's pretty similar to Majestic TF and it's counted in same way.

As the result you can get less than 1% of Pagerank from linking site.
Where Moz DA+PA and Ahrefs DR+UR will count it as 100%

And this "trust" it's only the one difference in Pagerank formula between old and the NEW Pagerank.
The old Pagerank is used by Moz PA, Ahrefs UR, Majestic CF but not contain the most important changes.

This is why Majestic is used by expirienced SEO's.
And this is why TF/CF is used, but it not applied to DA/PA, because it's without any sense divide whole site power by single page power.

So, your compare 4K TV to 720p TV haven't any relationship to the reality in SEO and mentioned metrics.
Especially when we talking about evaluating of NEW Pagerank for single url or for single anchor text.

And this is why I am said to OP, that only which authority will receive, will be in Ahrefs.
Thank you everybody for the info , i already registered the domain as a .com not the subdomain and the site is already live and going well, thank you.