WordPress.com doesn’t support AdSense, but we have our own ads program called WordAds – you can find out more here and find out if your site is eligible: https://wordads.co/
do you know if I can use ads on this theme if I switch to a non-wordpress host?
If you choose to run WordPress on your own hosting, you can integrate whatever ads you like; there are no restrictions. Poly isn’t available for self-hosted sites, so you’d need to select another theme. You can choose from any of the free themes in the WordPress.org directory: https://wordpress.org/themes/ – or find another premium theme you like.