I want to stop a subscription which is happening on a regular basis. How to do it?

It is easy.

1. Open the Settings app and tap on your name from the top.
2. Tap Subscriptions.
3. Under ACTIVE, tap on a subscription, and from the next screen, you can cancel it.

Note: Several free trails like Apple Arcade, Apple TV+ will end immediately. For most, you can keep enjoying them till the time you have paid for. Make sure you read the small text below the red cancel button (in step 3 above).

Hope it helps.

Start here: https://support.apple.com/en-us/HT202039