I am a mother and a pediatric, early childhood occupational therapist. We specialize in development (hitting gross/fine motor milestones and the like). Tummy time is working belly down against gravity to strengthen the muscles of the neck, back, and core. So important, not just for later crawling, but for lifelong stability and strength in the body. Wearing baby is a wonderful way to bond and snuggle, but is not tummy time. Like I said, belly down, against gravity (in a sling they are upright). Being prone (face down) on top of an adult is wonderful. If your baby is aversive to tummy time, try placing him/her against your chest and then leaning back so you are at a 45+ deg. angle. just hold on under their butt so it doesn't turn into an opportunity for your baby to stand up instead. Also, try not to belly flop your baby on to the floor!! Consider how you would feel if that's how someone put you down - you probably wouldn't like tummy time either! Consider lying down on the floor with your baby, and rolling him/her on to his side facing you....This is a nice introduction to not just being on his back.... Very gently, pull on the top hip to bring him on to his belly. If he fusses, give him a minute to figure it out. offer a favorite toy, or book, or your smiles. Then, when you feel he's done, get out! You will find that over time they will really enjoy this position. My daughter started rolling ON TO her tummy at 4 mos. because she didn't want to be on her back any more. She will spend 20+ minutes at a time (at 4.5 mos) iin tummy time, looking in a mirror I've set up for her and laughing!
Also also (sorry, I'm a bit sleep deprived, hence the nonsensical writing)! It is a common trap to assume that babies "want" to stand up very young. They want to be where you are -- if you are on the floor, they want to be on the floor. If you are high up in a chair, they probably want to be standing on your lap! In my experience, babies who spend a lot of time in standing have a more difficulty developing the skills needed for crawling because they get used to being upright. Give your baby the opportunity to move like a baby!