For getting a page indexed specifically, internal linking is the bigger lever, and the reason is that indexing starts with discovery. Before Google can index a page it has to find and crawl it, and internal links are its primary way of finding URLs in the first place. Google’s own people have said as much, that internal links are the leading method of URL discovery, with XML sitemaps a step behind. A page nobody links to internally is an orphan, and orphans are one of the most common reasons content never gets indexed, because the main road to reach them simply is not there.
Backlinks do a different main job. Their core contribution is authority, which is a ranking lever, how high a page competes once it is in the index. They can assist discovery too, since Google follows external links, but that is a side effect, not their primary role. If your question is whether a stuck page gets into the index, leaning on backlinks aims at the wrong target: you can pile authority on a page Google never reaches and it still will not get indexed.
It helps to keep the two jobs separate. Indexing is discovery, can Google find and crawl this page, and internal linking owns that. Ranking is authority, does this page deserve to be high, and backlinks weigh heavily there. Conflating them is what makes people chase links for an indexing problem that a basic internal link would have solved.
Treat this as a working judgment for the indexing job, not a precise weighting Google publishes, since Google hands out no ratio. But the practical order is clear, and it has the added advantage that internal links are entirely in your control while backlinks are not. So for a page that will not index, fix the internal path first: link to it from relevant pages that already get crawled, get it out of orphan status, and give Google a road in before you go hunting for outside links.