I hate to inject more photo advice because the pics are already pretty good.
Use a tripod definitely, but pay atention to aperture.
A small number is a large aperture, which renders the foreground and background out of focus. This is good because your subject is all we want in focus. And the subject doesn't have to be centered, we know it's the subject because it's in focus. Large apertures also require less light so you can use a faster shutter speed. Tripods are not necessary with a shutter speed at least as fast as the focal length, i.e. 1/60th or faster with a 50mm. But a tripod and using the self-timer is still nice for keeping the camera still during the shot.
Apertures wider than ~f2.8 can get a little soft, but it's good because it hides minor blemishes in the subject. A good lens will have a pleasant softness. A bad lens will just be soft. "Good" and "bad" here are unrelated to price or brand, but zoom lenses typically are just bad universally due to design, regardless of brand. Zooms perform best near the middle of their range in both aperture and zoom. A fixed length lens is superior at wider apertures. And a fixed 50mm/f2 is quite cheap.
A large number is a small aperture, which renders more of the scene in focus. This is bad because we don't care about anything but the subject. The background gets distracting when it should just fade from notice. A small aperture also requires more light and a longer shutter speed. And the smallest apertures are inherently fuzzy. Around f/8 is usually the sharpest, but for car pictures will result in too much foreground and background in focus. OK for scenic pics, not ok for pics of a single subject.
I think the first set of shots are great, but would be better with a larger aperture so the backgrounds are less distracting. But the scenes themselves are very nice, IMHO. The absence of direct lighting works well in the shots, I'd say. There are no harsh shadows but depth and detail are good. I'd say they are well exposed, also.