Internal linking is part of On Page optimization technique. It helps to get permanent ranking over search engines. You can’t do complete SEO without internal linking. It means connected your all web pages internally with contextual links.
While linked internally, you should be very careful. Mind relevance while linking internally. Internal links actually define path and let search engine crawlers know, what keyword should be targeted to get web page ranked.
Right method for accurate internal links:
Keyword<space> site:yourwebsite.com. By this syntax you will come to know most appropriate pages for internal linking. Then you should start internal linking.
Although making links are treated as Off Page optimization technique but here you are linking for your same site internally that’s why it is treated as a part of On Page optimization.