Arbitrum is a Layer 2 network that sits on top of the Ethereum Layer 1 network. Most ERC-20 tokens were created as a Layer 1 token. In order for that token to be available on Layer 2, someone (e.g. the development team) must bridge that token from Layer 1 to Layer 2.
You can find the most up to date list of available tokens in the Arbitrum token list. This list will continue to grow over time. When new tokens launch on Arbitrum, they will appear in the Uniswap app for you to trade.