16/3 vs 18/3 cable?

I’m likely gonna pull the trigger on the Bright Home Lights kit (https://www.brighthomelights.com/product/permanent-led-bundle/) and still doing my diligence and learning a ton (this is my first foray into lighting)

It’s unclear to me which gauge wire to use for the power injector runs and for when I need to run an extension. Based on my current install proposal, I don’t think any one run will be greater than 50’.