Well, it isn't super easy to give a magic formula for what will bottleneck. It depends on about a dozen different things. Just because you have a GPU bottleneck in one game doesn't mean it will exist in another, and the same goes for a CPU bottleneck.
Plainly stated a bottleneck is when one piece of the system prevents the rest of the system from hitting the performance it is capable of. Bottlenecks are easy to spot though. Just look for the component that is maxing out when the rest of the system still has unused resources.
Ideally when building a system you want a balanced system. So, I'll use my desktop as a good example. I have an i5 4590, RX 470, 16GB of RAM, and two midrange SSDs. In most games I'm GPU bottlnecked, but not in a major way. When my GPU is maxed out, my CPU is around 75-85% used in games that are graphics intensive. This allows my system to have additional CPU resources in case there is something that needs to happen in the background. I always like to have some extra CPU resources and to max out the GPU. If I hit a game where I'm CPU bottlenecked I crank the graphics settings until the GPU is working harder, which is an option for alleviating a CPU bottleneck.
Now, if I were to throw an RTX 2070 into my system I'd have an almost constant CPU bottleneck. The CPU just can't feed a 2070 enough and will max out. If I were to upgrade to a Ryzen 7 2700X, I'd max out the GPU but use less than half of what the CPU is capable of. Neither of those is a great case as I'd be wasting money for performance I could never use. A serious bottleneck is one where you waste money. It wouldn't be as bad if I were using the system for something outside of gaming that would take advantage of the extra resources though. This is why a 2700X with a RX 580 or something would make sense for someone who wants to do some gaming but mainly needs the PC for video editing. The CPU resources are being used, just not all for gaming, and when it is time to game the RX 580 can give everything it has to give even though the CPU sits mostly idle. It isn't a waste because the extra resources have a purpose.
Hopefully that wasn't confusing.