So if a PR consists of 20 commits, they review every single commit linearly without looking at the end result first?

Yes, and in some projects 20 commits is not even a big PR, more like "regular sized". The LKML's first page is now full of PRs with around 20 commits, here's a random one as an example: https://lore.kernel.org/netdev/20260408121252.2249051-1-dhow...

And here's a slightly smaller one which isn't about "miscellaneous fixes": https://lore.kernel.org/netdev/20260408122027.80303-1-xuanzh...

Some of these commits even get reviewed by different maintainers before being merged, which is common when a patchset touches several subsystems at once.