Custom Microprocessor Design for E-Cigarette: Sensors, USB-C, LED, Battery Monitoring

I'm looking to design and create a microprocessor for an e-cigarette business but electrical engineering is not my expertise. I have decent enough knowledge to know my way around a circuit board and create simple circuits but when getting into a microprocessor that is designed from scratch and customized, I'm a little lost. I need the processor to have a flow detector (might not need these, contemplating using button-activated instead), a volt meter to tell when the battery is low, a temperature sensor to cut off if the battery/coils get too hot, a LED to communicate to the user that the device is charging, in use, or dead, and a USB-C charging port. Any help/guidance with designing this would be great. Thank you!

This would I imagine be a very cost sensitive project, what sort of potential volume are you talking about.
The size will also be a critical factor, you need to see what space you have, to work out how you are going to pack it all in. You may be talking about ball grid parts to keep it small, which will dictate what parts you can use.
Firstly, decide what you definitely do need (flow detector?)
It is possible that you don't even need a micro, a comparator could handle the battery state, a charging IC the battery, and a temperature sensing IC and a few sensors the rest. A bit of discrete logic could handle the whole thing, which perhaps an 8 pin micro may do as a last resort.
If a micro is desirable, then you need to look at the I/O you need, it is going to be a low pin count, it would be preferable to handle the battery charging on it as well, to keep your chip count down. Depending on how you implement the whole thing, you would need some ADCs/comparators, and some general I/O for control. You need to sketch out the whole system in block diagram form, with all of the I/O functions, and work out what needs processor control, which will give you a function/pin count needed.
There are still some issues around the IC supply chain as well, although it does appear to be easing.
This is a very crude starting point, without more information it is not easy to go beyond generalities,
cheers,
Richard

Further to my earlier comment, in which I was suggesting that a microprocessor was probably not needed, here are a few more comments. In your application you are talking about using a mixed mode device, and there doesn't appear to be any need to do any complex processing, it is likely that a simple mixed mode ASIC is all you would need, rather than a processor, which does away with memory of various kinds, an execution unit, microcode, and all the rest of the support needed around a typical processor.
There are companies manufacturing ASICs, and if this is a large volume prospect, you would get quite a bit of support from their engineering teams to get you started. There are minimum order requirements of course.
Have you considered the possibility that you would be better off buying rather than starting from scratch, there are thousands of person years that have gone into producing the vast array of processors out there, you would be hard pressed to not find something already in the market that would do what you want.
Cheers,
Richard

Hello.
The project should not be complicated.
There is no need to use a microprocessor but a microcontroller is sufficient.
An Arduino or PIC MCU, with the corresponding firmware, sensors and actuators, is a good choice.
Hi
Giovanni

You might consider using an existing microcontroller platform such as Arduino or PIC MCU, and integrating the appropriate firmware and sensors to meet your requirements. This can save time and effort while also fulfilling your functional needs.
Best of luck to you!