Well, that assumes that all content is linked within ipfs too (which is the case here). Does that clarify it or should I explain further?

If the Spanish government wants to redirect or modify the contents of the page, then you would get a different hash than the one provided by Catalan officials.

How do IPFS links work, such that modification of what they point to modifies the hash of the document where the link is?

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.