The Knowledge Graph and Google My Business Profiles are Different Things
Knowledge panels are boxes that display information about (named) entities e.g. people, places, organizations, things that exist in the Knowledge Graph.
Knowledge panels for entities typically will include content obtained from a variety of different sources or domains. Ex: a knowledge panel for a corporation might include its logo from the company's website, and the founder(s) / current CEO from Wikipedia. If it's public company we might see its valuation and stock acronym.

A key distinction is that Knowledge Panels cannot be claimed or modified by the associated/represented entity. All of the information shown comes from Knowledge Graph data.
Google My Business Profiles, look similar to knowledge panels, but are specific to businesses that serve customers at a particular location or within a designated service area. Any entity can claim or create a Google My Business Profile and use it for marketing/advertising purposes.

If the profile is claimed, the information contained in a Google My Business Panel in the search results is added by the entity itself (or whoever claimed it), rather than being populated from information that exists in the Knowledge Graph.
If the profile is not claimed, the information will likely come from places like Yelp, MapQuest, or aggregators such as the Yext Network. According to Google:
Information in local listings is compiled from a variety of sources:
- Publicly-available information, such as crawled web content (e.g., information from a business’ official website)
- Licensed data from third parties
- Users who contribute factual information (such as addresses and phone numbers), and content (such as photos and reviews), including
business owners who claim local listings through Google My Business
- Information based on Google’s interactions with a local place or business
Searches that Trigger a Knowledge Panel
In the example above I triggered McDonald's Knowledge Panel by simply searching McDonalds
. A key distinction is the scope of its relevance. McDonalds is a globally relevant entity. There is factual information about it all over the place.
If I were to search for McDonalds near me
, I'd get a map result with the closest Mc Donald's locations to me. Those are most likely Google Business pages - maybe Mc Donald's has their own way of updating that data, but a lot of less massive franchises typically use Google My Business.
Additional note: If I search "what happens to nature in autumn" I get this:

That's not a "Knowledge Panel", but it is another Rich Result called the Featured Snippet.
How Exactly are Knowledge Panels Triggered?
Google has a patent on presenting data with search results and deciding when to show that data in a knowledge panel with those search results.

Essentially, when a search is made Google's systems will detect an entity in the search query and try to match it with a particular factual entity for which a knowledge panel is eligible to display with the search results.
You can learn more about the Knowledge Graph, how it's triggered, and the aforementioned patent here.
Does Google only uses structured data when it's for big brands?
In practice, yes. Think of the the knowledge graph like Wikipedia. If there's a Wikipedia page on it, there's very high chance it's in the knowledge graph. We're talking about widely recognized things that have a lot of widely available factual information on them. A small local business typically doesn't have that, unless it is famous for something.
I'm currently visiting family that Missouri - there's a popular corner bar/grill that everyone around here goes to. They sponsor little league teams, and are very locally relevant and well known. If I search for b halls
there you go. That's a Google My Business result. Looks similar!
