Urgent Help developing VTOL!

Hello everybody,
I’m trying to build an improved version of the vtol XK X450 or the E-flight convergence making it bigger, I’m new in this area and I need some explanations:
I will be controlling my brushless motors with a controller called KELLY which has a single input wire that gives the ESC 0-5 volt (0v = no rpm , 5v = max rpm), how can I convert the PWM signal coming from my flight controller to a stable voltage to give the ESC?

I was looking at the Pixhawk 4 as flight controller but I would need to know: What modules do I need and which function does every single module do? I’ve seen many schematics with many telem modules (why not just one?) and also a port called 12C (what is it for)?
I would also need my vehicle to be RC controlled from a ground station but also I would need to implement the possibility of controlling it directly from the flight controller with a joystick for example, would that be possible?

I’ve also seen videos about DIY drones that could carry people, how is that done? what kind of flight controllers and ESC do they use? how do they enable in-cockpit control with joysticks?

Thanks so much for the help!

I’m also searching for someone that I can eventually pay to help me develop this project as I really would like to see this project developed!