can I manually set aperture in Halide 2
No.
In Halide, you can manually set the focus, exposure, shutter speed and white balance, but the lenses are all fixed aperture. There is a portrait mode, but as with the stock camera app this is using software to fake different apertures.
does it really improve on photos taken with built in camera standard software
I think this is heavily dependent on what you are trying to do. Halide doesn't magically improve the photos you take - rather it gives you more choice to tinker with the variables listed above.
I think for the majority of users, the majority of the time, Halide provides little benefit. Making good judgements across the four variables - particularly exposure and shutter speed - is difficult, and often it's best to simple let phone do it automatically as it typically makes very good choices. Certainly if all you want is to take nice pictures of travel, pets, kids and so on to share with friends and family, there's no need to try a third party app.
Where Halide shines is for a small subset of users who enjoy making more unusual decisions - to shoot in a particular way in difficult light, to use a RAW image to reveal more detail (at the expense of making it more difficult to process), to produce a particular effect with a long exposure, to shoot with less ML processing (Photonic Engine/Deep Fusion) and so on.
I would also add there is a free 1 week trial if you want to try it for yourself.