I think by "infusions" you mean adding other "non-beer" flavors into your brews? If so, then this topic is widely covered on this forum, but since it is so broad, that may be why you're having trouble finding succinct answers. Here's a brief overview:
Yes, almost any flavor can be added into a brew as long as it doesn't infect the batch. Anything from fruit to wood, flowers to peanut butter have all been used. You can add these flavors anytime from sparge to bottling. For example, I have made a pumpkin ale by adding the pumpkin and spices at varying times. I've steeped pumpkin in my hot liquor tank before mashing in, I've added it to the mash, many people add it into a secondary vessel. I've added the spices during the boil, after primary fermentation has completed and on another brew I added it to my priming sugar when I bottled. This is only one type of beer, and I didn't list all the possible variations that could be used to infuse differet flavors. You can see how much flexibility you have.
The flavors do not have to be in liquid form. Then can be solid, liquid, powder, probably even gas if you could think of something that would work. You can add these flavor infusions at all points of the brewing/fermenting/conditioning phases.
I think you'll have more success in terms of finding info if you decide what flavor you want to infuse and search how to get it into your brew, or decide when you want to try to infuse a flavor, and search how to do it. Your one gallon experiments are how a lot of big breweries try test batches (look into dogfish head's beers for unusual flavor infused beers).
