Ask HN: How does Google search's subscribe to topic work?

I'm curious to understand the architecture behind the following feature:

1. Search for a topic 2. (I guess), if this topic is a node on the knowledge graph, allow user to subscribe to topic 3. user gets notification in google now about new webpages talking about the same topic

What's the architecture for this feature? 1. For every newly indexed webpage, extract keywords, send push notifications to all subscribers? 2. Do 1. periodically instead of real-time? 3. Constantly walk knowledge graph (which probably stores last-checked time), identify new (current-time>last-checked-time) pages, send notification to all subscribers?

Follow-up questions: 1. Is there a limit on the type of topics per user? Like nodes on knowledge graph only? 2. Is there a limit on the number of topics per user (or) on the global scale?