Links from subdomains to the main site, how will this affect the main site in terms of PS?

Question

Search Engine Optimization

Good afternoon. There is a site, now for many queries in the top, functionality expansion is required, but you don’t want to do it on the site itself, otherwise it will turn out to be a complete mess. I would like to do it on a subdomain (maybe even 2), the topic will remain the same, just a little with a bias towards another area in the same topic (construction). On the main site, I want to place a subdomain in the menu, that is, it will be like a regular button on the page, but with a transition to the subdomain. In general, this can be done, or it is not normal. And how it will be displayed on the main site in terms of PS? I'm a SEO noob, so don't judge too harshly))) I'd be glad for any expert advice. Thanks a lot for your time.
PS On the subdomain, I will also place a button on the main page of the main site. (I forgot to mention this).

Answer 1

99% will have no effect. Do it, don't be afraid :)