A doubt about the parameters to put in the API

The Subscriptions API returns paid subscribers
The Follows API returns followers

This is a URL to the paid subscriptions API

You called the wrong API, you called “Check user Subscription” - Reference | Twitch Developers

When you should of called “Get Broadcaster Subscriptions” - Reference | Twitch Developers

Get Broadcaster Subscriptions uses a token with channel:read:subscriptions where the token user ID = the broadcaster_id
Check User Subscription users a token with user:read:subscriptions where the token user ID = the user_id

So you had the right params, you just called the wrong API.

So

https://api.twitch.tv/helix/subscriptions?broadcaster_id=123&user_id=321

Will look up if 321 is subbed to 123 when using 123s token

And

https://api.twitch.tv/helix/subscriptions/user?broadcaster_id=123&user_id=321

Will look up if 123 is subbed to 321 when using 321s token