How to design a power distribution board?
But I don't know where to begin and key points.

I am sure noone here will build this circuit for you, but to help a fellow newbie start..
Reverse Current - Diodes, current flows in one directly (albiet slight leakage) https://www.allaboutcircuits.com/textbook/semiconductors/chpt-3/introduction-to-diodes-and-rectifiers/
Voltage Distribution - LDO vs Buck. Choose a voltage regulator to suit your current consumption. https://mindtribe.com/2013/06/ldo-or-buck/#:~:text=With%20an%20LDO%20design%2C%20voltage,whatever%20your%20Vin%20might%20be.&amp;text=With%20a%20buck%20regulator%2C%20power,components%20and%20their%20parasitic%20impedances.
What devices are being powered? What is their current draw? https://sciencing.com/calculate-amperage-draw-5328646.html
Most regulators will switch off if current exceeds but will heat up a circuit as heat is dissipated.
If building a PCB, review https://www.4pcb.com/trace-width-calculator.html to ensure your PCB tracks are wide enough to account for current pulled.
Further components to investigate are transistors/mosfet to turn off regulators when not used.
As a warning, please do your research and ensure components are sized appropriately to load including your battery. LIPO batteries can have disastrous effects if relevant protection circuitry is not implemented, as can be seen in unregulated vape mods drawing too much current too quickly.