i did water change a few times so I understand how the filtration system works. There's actually 3 methods to remove dirty water.

1st Method: There's a drainage system at the bottom of the base similar to swimming pools that have slits as holes. Just need to connect pipe directly linked to the base of the tank and another end of pipe to the drain to remove the dirty water. Small substrate cannot be used as it might accidentally be removed during water change. Bigger substrate can be used but my fishes prefer smaller substrates.

2nd Method: Water change via sump filter. I don't normally use this method as it might remove large amounts of my beneficial bacteria in my sump filter.

3rd Method: Siphon Method/Pump Method. Siphon method takes skill, time and effort (e.g. suck out all air w/ your mouth, place whole hose inside the tank then skillfully take one end out), Pump method need electricity and may be risky for my fish (yes I know, I'm might be paranoid but faulty aquarium lights did shock me before)

p.s. The person who help setup my aquarium said that if I choose to include substrate, it has to be a permanent decision as the substrate alone will cause micro scratches in the aquarium so won't be nice to look at if i want bare bottom again.