Contract business logic in RGB is defined through so-called schema; the specific RGB contract must implement some schema. One may think of a schema as a "class" definition in the OOP world; in that terms, specific RGB contracts are "class instances" created by the schema constructor (genesis operation). Such approach allows to separate role of contract developer (in RGB called schema developer) and contract issuer: the first one needs to be an experienced developer, while the second one is not required to know anything about coding or security at all. This also promotes re-use of common codebase by different issuers for the same typical use cases (like fungible assets), reducing the risk of mistakes.
RGB uses specially-designed virtual machine AluVM, which is Turing-complete in the same terms as EVM and WASM-based smart contracts (i.e. nearly computational universal, but is bound by number of operation steps, measured by gas consumption in Ethereum-like systems, and by accumulated computational complexity measure in case of AluVM).