How to pass a user ID with the Google Play Billing Library?

The documentation reads:

Starting on August 2, 2021, all new apps must use Billing Library version 3 or newer. By November 1, 2021, all updates to existing apps must use Billing Library version 3 or newer.

Which would be this dependency:

implementation "com.android.billingclient:billing:3.0.2"

How to pass something alike a user ID with the billing client?

One can pass along an accountId and profileId with the BillingFlowParams:

String accountId = "";
String profileId = "";

BillingFlowParams.Builder builder = BillingFlowParams.newBuilder().setSkuDetails(skuDetail);
if (this.accountId != null) {builder.setObfuscatedAccountId(accountId);}
if (this.profileId != null) {builder.setObfuscatedProfileId(profileId);}
BillingFlowParams billingFlowParams = builder.build();

Then on onPurchasesUpdated() one can be retrieve AccountIdentifiers from a Purchase:

@Override
public void onPurchasesUpdated(@NonNull BillingResult billingResult, @Nullable List<Purchase> items) {
    if (items != null) {
        for (Purchase item : items) {
            if (item.getPurchaseState() == Purchase.PurchaseState.PURCHASED) {
                ...
                AccountIdentifiers identifiers = item.getAccountIdentifiers();
                if (identifiers != null) {
                    String accountId = identifiers.getObfuscatedAccountId();
                    String profileId = identifiers.getObfuscatedProfileId();
                    ...
                }
            }
        }
    }
}