.png "ddmeltzer8"){kind=avatar}
You won't have to worry about TDP if your building a custom PC, well you will and you won't. The mainboard you purchase or plan to use already is configured to handle the hardware you plan to install onto it and TDP is assigned to ALL chips including CPU, GPU and all board system chips, which will be located on the specs of the board.
If your trying to replace the CPU with the newer then you might worry a little, because the socket might be locked to a certain wattage or if there isn't enough memory Space built into the board itself that is when you need to worry.
You dont mention what config but your question is based on purchasing a 3700x-CPU.
PCIe space registers are mapped to memory locations located on board based on specs, a fully unlocked main stream board from a known manufacture such as EVGA or MSI ect ... preference wont matter , will have a hardware listing of all parts for all configurations listed on the main websites along with listed amount of SPACE based on the chipsets <<< this determines over-all cooling and TDP. From how i understand it.
Finding the actual SPACE registered toward the entire board is going to difficult to find and might have to place actual phone calls to find out info, this is why so many people copy/paste builds.
And to think im knee deep in debt and went to college for this, shs
I think this is more were your question was asking. So, basically your CPU will be the major conflict when your considering TDP alongside the amount of hardware and the configurations it can handle at once, which most lower to mid range CPU's are limited way more than a person might think. So example if its running Up to just (1) config of 1-1x16 or 1- 2x8 or 1-x4+2x4 slots your TDP is going to be extremely limited and semi-locked wasn't really a option by a manufacture.
Most pre-builts today market is (1-1x16 + 1- 2x8) OR ( (1-1x16 + 1- x8 + another 1- x1) <=> Limited 
Why they are still using PCI x1 slots on new pre-builts , not sure
But allowing Max # of PCI Express Lanes 16 ( based on a newer xxx gen i7-7400 cpu) The AMD Ryzen 7 would be the AMD version of this , sort of.
I almost go off topic here because a GPU CPU and system chips all go hand in hand for TDP .... The TDP is not the maximum power consumed by processor, but it allows ((CPU ( cooler manufactures)) for VGA to size their GPU coolers.
Like what was first said, the actual info about all this never was explained to consumers, Main board manufactures like AmericanTrend and AMI or some other third-party getting in on the action can use the number to create some stuff ,lol i have to laugh how that came out but its about as true as it gets 
So calculated off wattage and the heat dissipation that is known and tested combined to give a rough estimate to start calculating to rest of the specs like tJunc and over all system performance based on max running times.
As far as using TDP to your advantage? Thats why people get into overclocking , which if thats your intent PLEASE link the board you plan to socket your new AMD 3700x into
Technicall you dont use TDP to your advantage , its a label placed on box to allow people to get a idea of what CPU's the board might take and vice-versa ... what board the CPU can be socketed to.
This is why people shell out tons of cash and all we can do is hope the manufactures all evolved, knew what the H they were doing. TDP= Power which is Wattage Max of CPU on a manufactures standard based on testing and manufacturing of the chip.
Default TDP is 65w and Unlocked for the 3700x 8 cores, this isnt your everyday consumer CPU.
What was your intent for the use of such a powerful CPU?
Pleas be sure to click the hyper-link HEAT in my reply, scroll down to Classical thermodynamics. That would be a good start to basic calculation of TDP and Heat generation.
But i do like the topic , best CPU question i heard in long time
