You're referring to PSD2 / OpenBanking?

What do you mean it's impossible to get access to?

If you're a start-up wanting to share your customers' financial data, you can make an API.

If you're a start-up wanting to get access to your (potential) customers' financial data from other institutions, you can get access through one of the many providers (Tink, GoCardless etc), or become a provider yourself with enough work.