Integrating Hyvä Themes with a checkout process in Magento 2 OpenSource depends on the type of checkout you want:

OPTION 1# Hyvä Checkout (Recommended)

This is the most recommended option for seamless integration with Hyvä Themes. It’s built with Magewire, a technology familiar to Magento developers, and offers a highly customizable checkout experience.

Here are some steps to integrate Hyvä Themes with a Hyvä checkout:

1. Go to Stores > Configuration > Hyvä Themes > Checkout in the admin configuration.
2. Select Hyvä Default for the Checkout setting in the General section.
3. Save the configuration.

Hyvä Checkout can also be installed using a license key from packagist.com. Here are some steps to do that:

1. Run the command “composer require hyva-themes/magento2-hyva-checkout”.
2. Disable Magento HTML minification in the configuration settings.
3. Execute tailwind to generate checkout styles.

OPTION 2# Luma Checkout

This is the default Magento checkout option. You can use it with Hyvä Themes by installing a module that allows it to fall back to a different theme on checkout pages. There are also third-party checkout extensions typically based on Luma that might work with Hyvä’s Luma fallback mechanism.

OPTION 3# React Checkout 

An open source community supported checkout for developer teams with React JS experience. This enables you to use the standard Magento checkout with any standard Magento checkout extension.

A list of more third party checkouts can be found in the Hyva’s Compatibility Module Tracker.