Which format is the best one overall… using a subdomain or a subfolder. Which will be the best one for search engine optimization and why did you select subdomain for your forums.
In terms of SEO it does not make much difference. If you link your forum from your blog and vice versa then everything is connected and Google will distribute same domain authority on your both property. They will trust your subdomain just like any sub folder.
Google does not trust all kind of subdomain, for example blogspot com are not connected to each other, and they have different publisher plus they use different Google search console account.
People use subdomain because it is easiest manage, sometimes when you try to install some additional software like forum or eCommerce store in seprate hosting service, so it won’t down your other site as well, when server have some issue(for example bandwidth load).
Don’t trust Moz article on that, many of pro webmaster criticize them, even john Mueller said many times it does not make any difference, but still they do not accept it, I mean why? It’s not any blackhat things that Google avoid to suggest. It’s simple thing and they already said many times, but still they did not accept it.
I agree with what @Goyllo said!
I needed a standalone domain for this forum, so I thought a sub-domain will be suitable for this. Moreover, at least for me, sub-folders look like a “page” in a website, so I didn’t opt for it.
SEO-wise, I dunno bro, Google may not actually give any specific preference for this I believe.
But, to be honest, it’s your personal preference at the end of the day.
Take a look at https://www.theverge.com/forums
They used a sub-folder for this. They are pretty reputed as well.