what is the role of SPI interface in this diagram ? will it do anything to LO ?

SPI is a communication interface.
I assume it's used to configure the IC.

What exactly is configurable by the SPI and how it's done - is probably described in the datasheet of the IC.
Did you read it?

Hi,

yes, most probably

--> Read sections 3.8 and 7 of the datasheet.

Klaus