Mesh MK63s router compatibility / cable modem

> Ive got an older netgear cable modem, [...]

   Has it a model number?  Look for "Model" on the product label.

> [...] I recently bought and installed the nighthawk mk63s system.
> [...]

   Why?  Is there some actual problem which you were trying to solve?

> [...] my existing modem. [...]

   Has it a maker and a model number?

> [...] So my question is do i need a WiFi 6 cable modem to get true
> WiFi 6? [...]

   A modem and a router are two different devices.  It's possible to get
both modem and router in one box.  You need one of each.  Without more
detailed descriptions than "an older" or "my existing", it's tough to
guess what your equipment might be.

> [...] Do i need a new Nighthawk cable modem? [...]

   For Xfinity, you need _a_ cable modem.

> [...] And does my cable modem still send out a WiFi signal?

   Wi-Fi is a router's job.  If your (unspecified) "my cable modem" is
actually a modem+router (not a simple modem), then its router section
could.