Both Bit Rate and Throughput measure the data transfer rate of a network connection, so both of them are measured by bit per seconds.

The main difference between Bit Rate and Throughput in a mobile network is that,

Bit Rate is the theoretical maximum data transfer rate of a network connection.

While Throughput is the practical data transfer rate of a network connection, that can be achieved in the real world.

This means that the Bit Rate is the "Theoretical" data transfer rate, while Throughput is the "Practical" data transfer rate.

So that Throughput is lower than the Bit rate, this is because Throughput is affected by different factors such as network congestion, device capabilities,...etc.