You may use one of the existing audiences or create a custom one. To create a custom one, click New audience and follow the on screen wizard.
Head over to your Google Ads account to use your audience in remarketing. Start by creating a new campaign and select the goal type you want.
Select the campaign type relevant for your use case.
Make sure to select Standard Display campaign and click Continue.
Go to Targeting and click on Audience Segments.
Then go to How they interacted with your business under Browse, and open the Website visitors menu. Select the audience you like to use and click Done.
You may select multiple audiences, and you create new ones in Google Analytics 4 to use here. You can also create your own in the Google Ads Audience Manager, but this will require additional event tags on your website.