Diamond press-ups target the tricep much more than a typical pressup, a normal pressup targets the shoulders and chest, however during the tendon building stage you’ll feel your arms being incorporated a lot, that might explain why you feel pressups they work the arms. If you want to work your biceps, I truly do suggest getting dumbells, and if you dont mind saving for a litle while as you are building tendon strength, get weight dumbbell, you can change the weight (2kg-32kg, 4kg increments) by rotating the bar like a motorbike. Its super approachable