You will find a list of all available applications in the Add-ons section at the bottom of the left-hand sidebar. They are organised into different categories to help you find the right integration faster.
Select the appropriate add-on and click on Install add-on. You will also see its price. Most integrations are free. It is also worth paying attention to the author – if you have any questions, you should contact them directly.
Then, enter your store name, click on Go to your account and on the next window confirm the installation.