The answer is that decentralization is in many ways considered a spectrum. We're a long ways away from being able to interact with web3 with no centralized points. As an example, you're reliant on your ISP, DNS, and the web of trust (involving tiers of certificate authorities) to even connect to a web3-enabled site from your browser. The most popular wallet for interacting with smart contracts in the browser is source-available, but not open source.
Of course, any interactions with the blockchain directly (calling methods of deployed smart contracts, or transferring tokens), are decentralized, but if you want to verify the source code of the contract you're interacting with, the most convenient way again involves relying on a centralized service (etherscan or similar for other blockchains), while the decentralized way would involve downloading the contract source yourself, compiling it, and comparing that with what's on the blockchain.
Of course, any interactions with the blockchain directly (calling methods of deployed smart contracts, or transferring tokens), are decentralized, but if you want to verify the source code of the contract you're interacting with, the most convenient way again involves relying on a centralized service (etherscan or similar for other blockchains), while the decentralized way would involve downloading the contract source yourself, compiling it, and comparing that with what's on the blockchain.