Found this simple circuit on youtube and I'd like to know the equation for how to vary R and C to change the flashing frequency.
Also, how to increase the current through the led with 3.3 V battery as power source to maximize brightness.

 

Depending upon what You expect this Flasher to do, this is a poor design.
This Circuit is discharging the Capacitor into the LED,
this "spikes" the LED with far more Current than it is rated for and will probably shorten it's life-span.
This is a "gimmick" Circuit made just to get Clicks on YouTube.
Use a "555" Chip instead, it is far more versatile.
.
.
.
Or use a standard 2-Transistor Multi-Vibrator-Circuit like this ............
Change the value of the 2- 100K Resistors to vary the Flash-Rate.
.

 

Difficult to tell as it isn't drawn properly, but isn't it meant to be a serial astable?
https://www.industrial-electronics.com/hobby-electron_mag_1979-03_into.html (about a third of the way down)
If so, it's a reasonable circuit but in need of some current limiting.