I've bought the subscription, but it's asking me to buy it again on my other devices. Do I need to make separate purchases for each device?

No, you don't. Please find the 'Restore purchase' link on the paywall. It's below the 'Subscribe Now' button. If you have multiple devices signed-in to the same account, Restore Purchase will allow you to extend your subscription to your other devices without having to pay again. 

Can I use the app on my Macbook?

As long as you're running macOS 11Big Sur or newer, you can download and install iPhone and iPad apps onto your Mac. There is no web version currently available, but we are working on providing one in the near future.

I want to cancel or refund my subscription, or convert a monthly subscription to an annual one. 

You can do this on your phone's Settings. You can find instructions for how to do this by searching 'manage the subscription from apple' in your language. Please find the English version here:  https://support.apple.com/en-us/HT202039 

Can I delete my account on the app? 

Yes, you can do this on the app you use. Please go to the 'Settings' or 'More' and find 'Delete my account' at the bottom of the page. For security purposes, you may be asked to log in again to confirm your account.