Why did Googlebot crawl my page but not index it? This 14-slide deck answers the question every webmaster asks when Search Console shows "Crawled – currently not indexed": a crawl is a fetch, not a verdict.
Act I is discovery and fetch (crawl budget, the 2 MB Googlebot fetch limit, the mobile crawler), Act II is parsing, rendering, canonicalization and evaluation, Act III is the indexing decision — with the difference between crawled but not indexed and Discovered – currently not indexed spelled out in Google's own words.