What are the differences between Channel-to-Ground & Channel-to-Channel isolation?
It sounds like you're talking about an oscilloscope or something similar. In that case, the channel-to-channel isolation describes the isolation between each channel and every other channel. The channel-to-ground isolation describes the isolation between each channel and ground. These are different, but related, concerns.

Channel-to-channel isolation describes how much voltage differential there can be between two inputs to the scope.

Channel-to-ground isolation describes how much voltage there can be between any input and the ground input to the scope, usually through the power connector.

The "techniques" involved aren't different at all. When two things are to be isolated, make sure there aren't any conductive paths between them, and make sure that all barriers between them (optocouplers, transformers, air distance, creepage distance, insulation, etc.) are rated for the isolation voltage you want.

Now, if all channels are isolated from each other, they must also all be isolated from ground; if they were all referenced to ground, they would all also be referenced to each other! It is, however, possible to have all channels isolated from ground while still being referenced to some other common node. This scenario can be useful, depending entirely on what it is you want to measure. If all the signals being measured are referenced to the same voltage, which is not earth, then your scope also having isolation to ground but not channel-to-channel would be fine.

As for crosstalk, that would require more details about what you're doing. I don't think there's a general answer there.