The new types of NVRAM use "FRAM" (ferroelectric RAM) https://en.wikipedia.org/wiki/Ferroelectric_RAM These do not use any batteries, the machine does not need to be powered on, etc etc. Think of it as just a magic way that without batteries, the new memory chip/adapter can retain all the high scores & game settings for 45+ years.

The easiest way to figure out what you need is to look up your game on IPDB.org and see the manufacturer / boardset information there (ie. Sega Whitestar). Sega being the manufacturer, Whitestar being the boardset. Then you can use the charts on the nvram product pages at PINITECH.com to figure out what RAM you would need.

Disclosure: The "Pinitech" website mentioned above is mine. I've tried to make it easy to figure out what your upgrade options are for nvram, where to locate existing RAM, what is an easy swap-out and what boardsets usually require having to desolder the existing RAM and install a socket. You're welcome to PM or email me if you'd like and I can help you figure things out further based on the games you'd want to upgrade.