Let me try to help with my understanding. A rough comparison of a blockchain is a database - only that steem isn't a typical databse as here each new block of it has a reference to its predecessor, forming a chain.

From an architectural perspective, the web applications have different layers, this is for better scalability, maintenance, development, support, etc.
Usually, the web apps have 3 layers

• database layer
• business logic layer
• presentation layer

Those layers could be hosted on the same server or cloud, or not.
For instance, if you trace route any of the steem frontends (steemit.com, upvu.org, moecki's version, etc.) you will see they are located in different parts of the world ;) So those sites(apps) you mention are Business+Presentation layers that use the STEEM blockchain as a database. So they are all off-chain, to answer your question. ;)

In theory, it is possible to store an app on the blockchain but then to execute it, still some external service will be needed to do so, also the blocks are limited in size and a bigger app would be very fragmented.

It's like building a car with Lego parts - it's possible but how good such a car would be? ;)

Another limitation is the STEEM blockchain is text-only.

Long time ago, in the previous century, lol, I worked as a developer for about ten years, on all the three types of layers - now such roles are called a "full-stack developer" but back then there wasn't such a title :)