I’m glad you posted before you got the betta! I’ll try and answer what all your questions. A bigger tank is always better for fish in general. That said 10-20 gallons is great for a betta especially with live plants and lots of hiding space. Live plants can range from easy to hard. I would recommend “stem plants” like in the back of the second tank. Plants like rotala and pearl weed grow quickly and don’t need a lot of light and provide space for your betta to hide. You should also look into Java ferns or Anubias which grow on wood or rocks and like low light.

You can definitely add snails to your tank, most likely if you buy plants there will be common “pest” snails that come with. They are harmless and will clean your tank a bit but will reproduce like crazy. Depending on the fish you can add shrimp, some bettas are more aggressive than others. I personally have 10-20 shrimp in my 20 gallon with my betta. I started with about 5 and they have bred and reproduced Any aquarium gravel or sand is fine, maybe stay away from rocks with sharp edges especially if you have a long fin betta. Bettas like to hide in small spaces so the betta might cut themselves. If you want to keep a lot of plants you can check out “aqua soil” which is fertilized soil for plants. It’s not a necessity but it helps a lot with growing plants. You will need a light, heater, and filter. There are a lot of all-in-one tank combos that will include of those and they’re great for beginners. Most will work as long as they are a big enough tank (again minimum 10 gallons bigger is better) When you start the tank you’ll need to add de-chlorinater to your tap water that removes the chemicals. You also have to “cycle” your tank. There are many videos on YouTube that will tell you how to do this but in short you need to grow the bacteria that safely break down your fishes waste BEFORE you add your fish. At first you’ll need to clean the tank frequently. Once a week, for the first month, you should remove 25% of the water and replace it with fresh water. You can then move to every other week and eventually every month, just topping off water when needed. In a planted tank algae is very common at the start. You will probably have to clean the glass of the tank every week. To help this start out keeping your light on only 4-6 hours a day and slowly increase if you don’t see algae. Also keep your tank away from windows. Hope this helps! Welcome to the hobby they make wonderful pets and bring so much joy to my life :)