The first time Ambitus starts, after the welcome, it offers a free trial: Start Free Trial. For 7 days everything in Ambitus works.
After the trial, Ambitus asks you to buy it once to carry on. There is no subscription. The price is shown on the purchase page and in Settings.
One purchase covers your iPhone, iPad, Mac and Apple Watch, and your family through Family Sharing.
Your Free Trial Has Ended comes up, with Buy Now and Restore Purchase. Until you buy, Ambitus stays on that page.
On a Mac, Quit Ambitus at the bottom of that page closes Ambitus.
Nothing of yours is lost. Your notes and lists are still in your own folder, where you can open them in the Finder or the Files app. Your tasks and events are still in Apple's Reminders and Calendar.
Settings ▸ General ▸ Ambitus shows the status, with Buy Ambitus and Restore Purchase. The status is one of:
The trial and the purchase belong to your Apple Account. On another device signed in to the same account, or after Ambitus has been deleted and installed again, Restore Purchase finds it.
People in your Family Sharing group can use Ambitus without buying it themselves. The free trial isn't shared: each person has their own.