Welcome to the forum and thanks for posting! Good on you for trying to improve the well-being of your fish; that’s the best way to learn about the hobby!
Everyone so far has given great advice. I will add a bit more. When you do your water changes, make sure to add the water condition (as mentioned above), which will remove chlorine/chloramine toxic to fish. Ensure you don‘t clean any fish equipment with soap (also toxic) or chemicals that aren’t aquarium safe (with some exceptions - just check first to be sure). It is also a good idea to heat the water to the same tempera as the tank before adding so that your fish don’t get a shock. This can be done easily with another heater.
The bubbles they are blowing may be a bubble nest, although normally male bettas form these more. If your fish aren’t having trouble swimming (such that they can swim around the tank, not just the surface, without issue), then it is probably nothing out of the norm, and you just have a quirky pair
Just be sure to keep an eye on how their fins and scales look regularly, as this can help detect illness or aggression early on. As others have said, it would be ideal to rehome one or separate them as, while female bettas can coexist, the tank isn’t large enough to house both.
Do you currently have a filter in your tank? If not, it could be a worthwhile to invest in a sponge filter. Don’t buy the cheap plastic filters that hang on the edge of the tank; these are generally poor quality. With gravel, it is a good habit to get into to vacuum during your weekly (or more frequent, while cycling with fish) water change. This can be done with a gravel vacuum, which you should be able to find online at reasonable prices. This isn’t the top of your concerns, though is needed in the long run (I have been looking at some recently, if you need help finding an appropriate one). Also, I am not sure if you did this, but when adding gravel (or any substrate) to a tank, it is good to rinse it before adding, since this removes fine sediments that can cloud the water.
I know it must seem daunting now, with so much to learn and do, but it does get easier - I know, since I was in your position years ago. If you need somewhere to start: get your test kit and monitor your water and keep an eye on fish behaviour and appearances. These, combined, should let you pick up issues quickly. Good luck, and if you have any other questions or concerns, feel free to ask.