How to select step-down converter chip?
If you are designing for commercial use and volume production the usual way is to just ask your distributor (eg. Arrow). Their sales guys will give you many options (logistics and availability are equally important as technical parameters). Of course they will move a finger only if you are thinking of mass production.

If you are designing for hobby use:


Go to chipmaker's website and use their part selector. Every manufacturer has one. Examples:


TI
Linear
ST
NXP
ON semi

Input your design requirements and narrow down until you have a manageable list of parts.
Look up these parts at the place you want to buy from (Mouser, Farnell, Aliexpress...) to see their availability and pricing.
Once you have a part selected, download and read its datasheet, especially the part with typical application and recommended layout. Check required external components.
Rinse and repeat