How can I use Ethers.js to approve a token on the Frontend

I have been creating a test Dapp but for approvals, I was doing it on Remix
Initially, I was trying to call the approve function from my Smart contract, then It was explained to me why it won't work that way

And someone told me I could do that through the frontend
The ones I have seen on StackOverflow, isn't giving me the exact response I am looking for

My DEX Dapp might not know which token wants to be swapped, so how does it accesses the approval function for that specific token, in order to grant spending rights to my contract?