I’m new to high-end- high-tech flashlights, the flashlights I do have take 18650 batteries, where do these 20700 batteries fall into place ?
Had trouble with that title. You mean a “Sanyo 20700A - 30 A / 3100 mAh”. Well you could look it up Wiki / Google…
Battery Mooch
This…Sanyo NCR2070C 3500mAh 30A 20700 Battery - 18650 Battery Store
Will this fit in a flashlight that takes a 18650
No, unless it’s specified to take 20700/21700s.
20700s//21700s are bigger than 18650s.
Why?
18650: 18mm D, 65mm H
20700: 20mm D, 70mm H
21700: 21mm D, 70mm H
So, you need a light like this, which does support 21700s:
https://www.amazon.com/Tactical-Flashlight-Battery-Charger-Camping/dp/B07GS21VYR/
Are they necessarily better than 18650?
Yes, as they can have much higher capacity, high drain capability.
As an example, the Samsung 50E is a slightly higher energy dense Samsung 35E 18650 in a 21700 size can.
Just ordered 2 of those 'C's - looks good but think they fall short of the 30T and 40T - worth looking at though.