My smallest tank was 10g, I'm not sure I'd go smaller, even for "just" a betta. I really liked my planted unheated cherry shrimp 10g. My smallest tank these days is a 20g long.
Lots of options for either. & no matter where you start in this hobby, you'll want to try something else eventually...or you'll give up at the first fish death or algae outbreak, many do... & you may be able to pick up a used tank set up cheap at a garage sale. If you can't commit 30 minutes a week or 2 to maintenance, you're not ready for pets of any kind. Sorry, I tend to be blunt but realistic.
I do like a filter for water movement. Heaters can be optional, but bettas like it warmer than I keep my house in winter. Live plants are nice & a few are very low light & low maintenance...bettas love them, lol.
It's great you're researching before getting a tank! When you narrow down what you might like, ask us again. Here's a few more to consider for a smaller tank: pygmy corydoras, micro rasboras, least killifish, ember tetras, honey gouramis, dwarf rainbow fish...there are lots!