Backlinks should always be sourced manually.

Here is a list of such a service providers, https://www.fiverr.com/gigs/dutch-backlinks

Then it becomes a business requirement to have separate websites.

Then it becomes a business requirement to have separate websites.

Also, if they are 2 completely different businesses?

Besides the above mentioned, use such a content on your webpage so that people can link your webpage. This works wonderfully.

I recommend to maintain only one website rather than multiple websites. With one SEO effort, benefits will be shared with all the pages on the website.

When a website links an another website then the linking website passes the link juice to the linked website irrespective of whether a link is declared dofollow or nofoll...

Subdomains and domains are treated as two different properties by search engines. Both should have separate robots.txt files etc. As @ms mentioned, if the content on bot...

Yes, it can harm SEO not only SEO but the overall performance of your website on Google. As your website has already existed with different extensions Google can get easi...