What are good exercises to build up a horse's topline on the ground (walk, trot, canter) OR riding (walk only)

Pole work and backing also doing belly lifts (scratch their stomach until you see their back actually lift, keep scratching for like 5 more seconds, stop give 5 seconds rest and repeat only do like 5-10 reps to start)
While riding do long and low. Make your reins long and then spread your hands out as far as you can and lightly wiggle the bit until your horse brings his head down and put forward. The goal is to have his head under his knees. This will take time because he may not be used to his center of gravity moving to the front where it should be. Hope this makes sense!