Any contract address can be found on network explorer. Each network has its own explorer. Example on how to find a token contract address
Binance Smart Chain
For Ethereum, the explorer is https://etherscan.io/
If you will search for USDT, you will find the token contract address there:
Lets search for the USDT on BSC (Binance Smart Chain), the explorer is https://bscscan.com/
Same token have different contract address on other chains.
Another way to find it, is by using one of the biggest price tracking site, CoinGecko, they have the contract address for all listed tokens. Example:
Cake Contract address
Also, you can see there, on what explorer this token can be found. P.S. We have integrated the CoinGecko API calls and tickers into a great Bubble plugin, take a look:

Video tutorial by one of plugin users

All MetaMask features work only inside the integrated browser in the MetaMask app. If the user loads the app in the default browser, you can use the deep link to open the MetaMask app on the user's device (if he has it installed).
As an alternative, you can use the WalletConnect plugin, it can connect to MetaMask from any mobile browser.
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