To find items on Polygon, make sure to check "Polygon" under the "Chains" section on the left panel.
If you want to access through opensea-js API, unfortunately it is not supported yet (as of late 2021). See https://github.com/ProjectOpenSea/opensea-js/issues/72
But there is always a workaround if you are a hacky person.
You can analyze the Fetch/XHR request under the Network tab of your browser's dev console. This is an example query that searches for "bored ape" on the polygon network. You can see that it's making POST requests to the GraphQL api endpoint https://api.opensea.io/graphql/ while the page is loading. I think that's enough hint for anyone with webdev experience.