Some fitness trackers are contain a lot more processing power
and runs at 16mhz when not in low power mode.
Same "processing power" as arduino/atmega chips, the atmega328 which mostly used my UNO
can run at max 20Mhz so I don't see you point with processing power 
The point of this is not to buy and assemble but to buy and build on your own.
Well not my topic so I can't tell what he really needs because he doesn't tell much about it.
D.60
I don't like smart watches. Of course I had some of them: Apple watches, Samsung and even Xiaomi. They have a lot of mistakes and think that don't look nice. For me the best choice is Rolex clone. I don't have 15k dollars to buy original watches, but I would like to wear only good ones. On perfectrolex.io I buy all my favorite watches, they are made in Japan and think that even those who deal watches won't distinguish them from original. Here you can find even limited collections. I have left the link for you above
HopiePT:
Thank you for your answers.I can found many smartwatches that run in android but they are too expensive, what platforms are you talking about?
There isn't any lcd touch screen outthere to connect arduino? Not in the size you want for a watch
If i do a working prototype, can i send it to a manufacture and compress it to small size? no, they just manufacture it, not design it
There are no ready made Arduino watch kits available that can do what you want. The research, development, and time spent building, programming, and debugging for the project you want to do will end up being much more expensive than just buying a smartwatch.
However, if you want the challenge, and have the desire to learn many new skills along the way, and are not afraid to spend lots of money on specialized tools and many different components to see what works and what doesn't, then try and build an arduino smartwatch.
Here is my attempt(s) at making an arduino watch:
https://forum.arduino.cc/index.php?topic=592075