The shuttle SRB's provided most of the thrust at launch. But the RS-25 liquid engines were used because they could be throttled. The RS-25 liquid engines were started about 3 seconds prior to igniting the solid boosters. This allowed the liquid engine turbopumps to spool up and produce thrust. If you watch a video of the RS-25 engines starting up, you'll see an initial spray of liquid propellants just before the engines ignite.