Two chains, zero shared visibility
A blockchain's entire security model is self-contained. Validators or miners on Ethereum agree on Ethereum's state by running Ethereum's consensus rules and checking each other's work. They have no built-in mechanism to observe, verify, or care about what happened on Solana, Avalanche, or any other independent chain. This is true even though both chains are public and both produce cryptographically verifiable state. Verifiability within a chain does not translate into verifiability across chains, because there is no protocol connecting the two consensus processes. Each chain is an island of trust that ends precisely at its own validator set.
This is fundamentally different from the relationship between a rollup and its underlying L1, which this site already covers. A rollup posts its data and, in the validity-proof case, cryptographic proofs of correct execution directly onto the L1, so the L1's own consensus can verify the rollup's state as part of verifying itself. Two independent L1s share no such relationship. If a contract on Ethereum needs to know that a user really did lock 10 tokens on Solana, nothing in Ethereum's protocol can check that fact directly. Some third party, whether software, hardware, or a set of humans, has to watch Solana, form an opinion about what happened, and carry that opinion across the gap. That hand-off is the entire cross-chain problem in one sentence, and everything else in this course is really just different ways of building that hand-off.