Welcome to the forum from another newbie. To start, beautiful tank. What are you looking to stock?
The holes in the bottom are pretty common in reef tanks, but I am seeing more freshwater people leverage them. A sump is the easy option for using them, but many have plumbed canister filters to those bottom holes as well. Something I myself have considered doing when shopping for tanks that are predrilled.
They are also pretty easy to close. Just get some squares of glass, atleast 2" wider than the hole. Silicone the heck out of them and place the glass patches inside the tank, so the water pressures helps keep it all sealed. Once you add substrate, you'll never see them again.
With all that said, the holes are for filtration. Think about stocking and setup. Then figure out what filtration will work best.
Hope this helps.
-55g Tropical Community
-10g Planted Beta
-5.5g Planted Shrimp (Neo)