How to find CPU by needed parameters?
You can search on Digikey.

Although, USB 3.0, SATA 3.0 we will need some sort of good CPU which will have external RAM.

This is high speed, high density, complex, BGA, length and impedance matching routing and this is, by far, not something you should try as a beginner in PCB layout and you would probably not succeed.

This requires a lot of knowledge, expensive software, expensive testing hardware and lot of experience and usually is done by a whole team of engineer.

Check if you can lower down the specs, USB 2.0, SD Card is okay with a normal microcontrôleur.

If you really need to have those sort of specs, you can check on the Raspberry type of devices (although no USB 3.0 and I'm not sure about SATA).

This is what CPU routing looks like, and this is lower spec than you require


Source



You better find a board that is already made.

Here is one that could fit your requirements, although it uses M.2 for the drive.