Two trendy areas in finance — fintech and private credit — are coming together in a new multibillion-dollar joint venture. 

Affirm Holdings is getting its largest-ever capital commitment with a new partnership from private credit firm Sixth Street, which is investing in $4 billion worth of loans over the course of three years. 

Sixth Street is committing capital upfront for Affirm to underwrite short-term installment loans, between four- and six-month timeframes. Once paid back, the…