Think of a block device as a blank warehouse, a filesystem as the shelving layout you install inside it, and a mount point as the door you hang on the front. Without the shelving nobody can find anything. Without the door, nobody can walk in.
Two filesystems dominate RHEL 10: XFS (the default, journaled, great for large files, online-growable but not shrinkable) and ext4 (older, widely supported, supports shrink, slightly simpler on-disk).
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