if i give backlinks to my main root domain, will my posts benefit from it too and rank high or only the main page?
You need to diversify your incoming links so I'd go for a mix of the main site and posts. Most people when they link to you on their own link to the home page so I'd put more emphasis on the posts if you are doing this yourself.
Build links to homepage can help boosting both homepage and inner pages. However the weight will gives more to homepage over inner pages for its targeted keywords. The ranking of inner pages is depend on how authority on root domain, not solely on number of links pointing to its root. Build links to inner pages (aka. Deep Link Building) is a plus. It could give more chance for inner pages to have some rank compared to just having links from homepage. Links from homepage is considered as a self-voted so it will not have much trust compared to links from third party. Deep link building to many pages enough will also help your overall website got more authority and weights too. It does mean increasing in ranking for both homepage and all pages under (even pages that not having links from another website pointing to it). Conclusion - Build links to both homepage and inner pages is a good strategy. It would be better if you can target toward inner pages over homepage. Most of websites homepage are tend to get natural links to them easier compared to inner pages. The deep links building to inner pages will help increase your website backlink profile and make them more natural anyway.
it depends one number of contents... if you have huge number of quality content then if you just get backlinks for main post all the inner post will automatically get high rank and also as days goes on your inner pages too get backlinks from other sites automatically....
If you build back links for homepage, then mainly your homepage will be benefited from it. This will also be benefited for inner pages as well, but they will get a very small benefit from whole. So, it is advised to build back links for inner pages as well, if you are targeting any keywords on them.
Google love Optimization in a organize way like Blogging or Blog Linking, Smart Linking etc.. You can think for good backlinks specially Blog-links for inner pages, its really work.
Your best bet is to build backlinka to both the main page and to posts. Deep linking is great for your SEO. ~Russ