Where to start when designing a custom PCB as per requirements?
This is likely more involved than you think it is.

Streaming video is not trivial. In particular, streaming video with a microcontroller is not typically done, unless you are talking about very small resolutions. The throughput is simply insufficient.

To stream video, you will typically want an applications processor, which is a step above a microcontroller. It's a more powerful processor with heavy duty peripherals and much more RAM (you'll need quite a bit of RAM for a framebuffer). For instance, compare an Atmel ATmega32u4 (a microcontroller) with a TI AM3358 (a microprocessor). They are very different beasts. Even with a bigger microcontroller like an ST Micro STM32F429 (which even has a camera interface), streaming is going to be very challenging, if it's even possible.

At a minimum, you will need an external RAM, a fast camera interface, and a fast pipe to your WiFi chipset (which is a whole other can of worms).

In short, this is a very involved project, and the board design is only part of it: you will need to do all the design work (component selection, schematic capture, layout, DFM, etc) as well as firmware development (there is quite a lot of embedded firmware required). This requires extensive knowledge of many different levels of engineering: probably MIPI for camera to processor, embedded firmware development, extensive hardware engineering know-how, network protocols and network stack development, PCB design, ....you get the point. As others have stated, you are much better off using COTS parts.