First off, you have generated four cases with similar topics. In the future, please sum up your questions in one ticket and don't generate multiple ones, as that won't get you help any faster.
The noise figure on the nRF21540 is lower because the nRF21540 is a dedicated front-end module that does not have multiple peripherals causing additional noise. For details on the technology in the nRF21540, please read the product specification thoroughly, and ask specific questions if you have any.
How less number of peripherals help in reducing noise figure and it is affecting SNR at output or SNR at input as noise figure is the ratio of SNR at input and SNR at output?
The nRF52 series devices has multiple other functionalities as well, and are not only a radio, which will affect the output somewhat. I don't understand the rest of the question. Ketil has asked you to rephrase here.
