Decentralized Web - Q&A

What are the ways that decentralised apps can interact with one another? How does Ethereum's solution limit such interaction? What are the limits of Ethereum in terms of interoperability?

In general, Dapp-s call blockchains.

Explained in a simple way, the usual way to interact is for one Dapp to write something on the blockchain, and the other Dapp-s to read it.

As I wrote in my answers to other questions the public Ethereum mainnet is slow, overcrowded, often hacked, and centralized.