Please some direction

Hello there. Im a newbie and self learner. Have read a lot of tutorials, discussions and links like material.io from Google. Point being made, i didnt find some informations and if someone could help i would be very thankful.

First, can someone point some link or tutorial that gives me the right sizes that i shoul be using for toolbars, navigations bars and icons? each website says one thing. For android is 56 for toolbar i guess, for iphone is 49? im really having trouble to understand this in translating to the design, it is just using these sizes at the right panel?. One other thing, the grid is 8px each litte square? so for an android mobile app three squares gives the 24pt asked for icons? should i consider a px as pt in XD always? the measure unities from the right panel are all pixels?

If someone can give the straight number from the right panel for icons, toolbars and etc for both android and iOS would be great.

Secondly, is there a problem starting a design with a bigger screen like the iphone plus? should i start with the smallest screen and then export at different sizes? or can i scale down after?

Thank you very much for any input.