Because more people (allegedly) voted for Biden than Trump in the state-wide presidential election, but more voted for the republican than the democrat in a few local congressional district elections.

This is quite normal and even though there was fraud in the 2020 election (which undoubtedly cost Trump the presidency) that situation of red districts in a blue state does not automatically indicate fraud. Rather, it's just something that can happen with how the Framers of the constitution set up our electoral system.