Apache Redirect 301 /old /new is a prefix match. nginx wants a rewrite with a capture for the rest of the path, or a return 301. RewriteRule with [R=301,L] is closer to rewrite ... permanent. RewriteEngine On is not a thing you translate; nginx is already rewriting.
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Flags that have a mapping
R / R=301 become redirect or permanent. L is last. NC can be a (?i) prefix because nginx rewrite is PCRE. QSA is $is_args$args. HTTPS-off plus a host rewrite is an if ($scheme = http) / if ($host ~* ^www\.(.+)$) pair, not a mystery.
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The .htaccess to nginx converter maps those directives and lists everything else as skipped: Options, Deny, Header, unknown flags. Skipped is the feature. A silent drop is how you lose a decade of marketing URLs on cutover night.
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After the redirects, the crawlers
When the host moves, robots.txt and the sitemap still need to be valid on the new origin. That is a separate pass with the robots.txt and sitemap validator. Do not put the old Apache ErrorDocument 404 into nginx as a rewrite loop.
Test three URLs by hand after deploy: an exact old path, a prefix match with a tail, and a www-to-apex HTTPS hop. If those three work, the rest of a typical blog .htaccess usually does.
