Logical Increments is a PC part picker and guide website that lets you choose parts for your PC and virtually build a PC to see how well will it perform.
You have the option to build your PC by selecting its different parts and components. Compatibility is another major factor and you can also find out if the parts are compatible or not by pairing them up.
If you build a virtual PC that is compatible and seems to be good for you then you can buy the listed parts and components right from there as each part that is listed there also has a buyable link from which you can but that part/component.
Logical Increments has a list of pre-made PC Setups already build up that you can select from, and also make adjustments to them as you like. There are some pre-made budgets builds which are affordable, but are weak specs-wise, while there are some heavy builds that are best for gaming performances.
If you are looking for PC designed specifically for a single game only and optimized for that game, then you can check out the Game Builds section from the website where they have listed some of the most popular games along with the specifications required for optimum gaming performance.
Logical Increments also has guides available for how to build and assemble a PC if you are new to all this and it also has specific guides that will help you build a PC for a specific purpose such as gaming, or photo editing, video editing, etc.