Why there is a PLL next to VCO in Siglent SSA3021X ?

[jimon]

Noobie here, so if you watched you might notice some PLL business next to main VCO.

As I understand VCO generates first LO, and it's feed to first mixer, but than there is coupling to input (or is it output) of PLL next to VCO. Why do they have it there? To lock main VCO to 10 MHz ref somehow?
There is also second LO in bottom right corner, which is free running(?)/locked to main 10MHz ref(?) which feeds second (1:1) and third mixers (4:1).

Another question - tracking generator there also have a separate PLL? As I understand we just need to mix first LO down to spectrum's analyzer input working frequency. Does it mean that tracking generator PLL always generate same frequency? aka 1st_LO_freq_range - some_const_freq = input_freq_range

PS. Don't really understand that business with 3rd mixer, why it's needed? And how the heck does Siglent achieve -120dB-ish noise floor when even RF switches there are rated for -40-60 dB coupling?

Very thanks!