General design tips
Designing a flex PCB is no different to designing a regular PCB in terms of how you go about designing it.
Avoid using right angle corners, use curves, to reduce strain on the copper that could lead to it cracking
Avoid using solid copper regions, use cross hatch instead.
Components on flex PCB’s
Yep, all sorts of SMD components can be fitted to flex PCB’s.
Rigid Flex PCB’s
This is a flex PCB that also has rigid sections. I can be as simple as a rigid PCB that has a flex section which comes off it to connect to something else, or more commonly several rigid sections of PCB with flex sections linking them allowing to be fitted into complex shapes in the final product without the need for FFC connectors and FFC cables to link them.
A common way to design these is for the flex PCB to be the middle 2 layers of a 4 layer design, so the rigid sections are 4 layers and the flex sections are 2 layers.
You can have the flex on the outside, although there can be usability issues, but the manufacturer will build it however you want.