welcome to HCM.
there is definitely a relationship between tight underwear or pants on the testicles & sperms in a relative manner. Nature has designed the testicles to be outside the core body temperature of ~2 degree lesser. So, if there is a tight pant or underwear, the testicles come in close contact with the perineum
& the temperature may be altered, which may have an adverse effect on sperm production & motility.
& regarding the Q-2, it is not mentioned anywhere in the literature that it is a must for an underwear. however it is adviced to have a well fitting (neither too tight nor too loose) underwear, to avoid untoward injuries to a certain extent only.