How does l testicular v. connecting to l renal v. at 90* (as opposed r- less than 90* connection to ivc) lead to l varicocele? (varicocele-wikipedia)

Blood flow: It all has to do with the ease of blood return. When blood returns at acute angles, this is simpler than curves and turns. Just like driving around a corner, the sharper the corner, the more braking you have to do. When blood flow slows, varicosities are formed and you get a varicocele or collection of engorged veins.

Pressure gradient: Amongst a number of potential theories and explanations for a varicocele, this is a strong one. The gonadal vein enters the renal vein at a 90% angle only on the left side. This is believed to create a straight column of pressure that enhanced by gravity, leads or assists in the formation of the dilated venous plexus. A varicocele is by far, more common on the left.

Venous drainage: There is an increased risk of left-sided varicocele because of the angle of drainage from the left testicular vein, just like you mentioned. Because the angle is sharper, it is more difficult for blood to drain through (as opposed to the right side which is a smoother trajectory). Therefore, blood will pool backwards into the testicles resulting in varicocele.