I wanted to design visuals for google hub. So I was searching for something like APL(alexa presentation language). How can we design visual in Google actions.
You cannot create your own visuals for Google hub using Actions on Google.
You can however use the existing response components made available to you such as:
Simple Response, Rich response, Basic card, Browsing carousel, Suggestion chips, Table cards, Lists and Carousel.
Some of these response types allow you to include title, Images, media clips(.mp3), displayed text, spoken text. etc so you can customise the look to an extent.
As per google responses document (https://developers.google.com/actions/assistant/responses)
The look and feel is outlined in the Visual user experience UI Sticker sheet published by Google (https://dl.google.com/devsite/actions/sticker-sheet.zip).
You will be restricted to this look and feel.