Building E-Bike system
For starters, extremely broad, general questions are generally going to yield low-quality answers on Electronics StackExchange. But I think it is useful to step back and look at the big picture involved with making a system like this since it covers different engineering domains and specializations. It may also help you understand how much goes into this.
You'll need to start by coming up with product specifications for your E-bike, chiefly the maximum acceleration you'll want to obtain, the maximum weight of the design (including the rider). From there, you can calculate the power you'll need to accelerate the mass to the given speed (1/2 mv^2 crap), which will help you select your motor.
Once you've selected your motor, you can design the power bridge for it, which will be three half-bridge (half H-bridge) drivers — one to drive each of the three phases of your BLDC motor. For smaller loads, there are single-chip motor driver ICs that have an integrated power stage, but I'm fairly confident you'll be drawing enough power to necessitate building your bridge out of discrete FETs and using a gate driver IC to control those FETs.
Before selecting your microcontroller, you'll have to decide how to commutate it (trapezoidal or sinusoidal field-oriented control — FOC — or something more esoteric), and your rotor position sensing strategy (6-step Hall-effect, encoder, or sensorless with estimators).
In the simplest 6-step hall-sensor-based commutation method, you can get away with a very simple microcontroller (or even an off-the-shelf motor driver chip with built-in commutation), but for sensorless FOC, a microcontroller with a floating-point unit will make development easier.
Then you'll need to work on the battery side of things. Calculate your peak power output, desired battery life, and bus voltage. From there, select your Li-Ion cell arrangement, which will let you design the battery management system (charger and cell balancer) that go into the project.
There's a lot going on! Hope this helps you at least understand the complexity of a project like this.