Build Transaction: To build the transaction, cardano-serialization-lib library is the best library on Cardano, you can use it in both in the frontend and backend.
Concern: this library is wrote in rust, the package for the frontend is wasm(web assembly) package, which will be harder to implement it on landing website
Solution: Build a tool(website) that have ability to compose the transcation, the other website can just insert simple javascript function inside its website, and able to use this tool to build the transaction
Website Integration (Sending ADA)
HTML
<buttontype="button" onclick="handleWallet()">
Send ADA
</button>