It’s probably safest to try and narrow the number of variables you work with at once as you learn.
LED brightness can be influenced by many things, but if you are trying to wrap your head around drivers then the only ones you should be concerned with are amps and volts. As it turns out that’s a tricky relationship to explain to those who are not familiar with it: the relationship of voltage to amp draw is non-linear for LEDs. Take a look at these two example graphs in djozz’s post #51:
As you can see, increasing voltage a little bit will increase current a lot. Meaning that LEDs are very sensitive to voltage. We typically use drivers which allow us to establish a set current and then let the driver handle lowering the voltage to the appropriate level.
In your example earlier you had 7.4v input into a driver, as you can see this is not an acceptable output voltage for an XM-L2 or XP-G2. Typically a buck driver would step that down at something like 80% efficiency. So 10W input would net 8W output. As long as there is enough energy available the driver will typically be monitoring and maintaining the output current. Every other number will be dependent on that output current in some way.