A 5 gallon isn't big for a betta fish, no worries! Personally I think the smallest they should be in is a 10, but a 5 can work, as long as you take very good care of it! The only issues that can arise with the tank are if it's too tall (bettas can struggle to get to the surface for air) or if the filter creates too much current (stresses them out because they have to put more energy into swimming.)
It sounds like you're still starting out with this fish-keeping thing. Before you ever get any other fish, please do some more research before doing anything!! Fish are very delicate creatures and can be extremely difficult to care for. And don't get your info from petco or petsmart employees, they're usually very wrong about what advice they give out; I recommend checking out FishLore, Wetwebmedia, and tropicalfishkeeping. You'll need to learn important things like cycling a tank, how to maintain a tank, water parameters, and diet of your fish.
Id also recommend only using silk or real plants with your betta (preferrably silk for now, since live plants are also expensive and difficult to care for) Plastic plants can very easily rip a bettas fins and leave them open to infection. Sorry for the essay, but I just want to let you know what you're getting into... I hope your fish are still doing allright, and I wish you the best of luck!

I don't think u should add fish in the future. U need a 10g for that. But I'm sure your betta is happy in his new home. Good video!

Am sure your betta is much more happy in this tank. The tank turned out good.

They actually need filters. Clean the filter with water first and it won't make the tank dirty. Filtration is an important part of Beta fish environment.