IPFS is immutable and hash addressed, so 'modifying' something yields a new address. Existing documents that point to the modified thing will still point to the old version, unless their links change as well. Effectively, sharing a hash address to the main page is sharing a snapshot of the site.

It's essentially the same technique as git.

There are two types of links in ipfs. Static content that works by hash of the content. Dynamic content that is signed by the owner of the bucket.... I think I haven't looked at it for months

You use IPNS to give a static name and change the root hash of that name when you update content.