SRIs are what they are - they primarily inhibit serotonin transporters, which increases extracellular serotonin concentrations. Serotonin is itself an agonist at serotonin receptors, it's a ligand. Quetiapine is a mixed partial agonist at serotonin autoreceptors and an antagonist at others, the antagonist action is of competitive nature, it's concentration dependent - in short quetiapine is overall a serotonin inhibitor. Which is more often than not good news for bipolar therapy as stand-alone SRI therapies tend to be activating as opposed to their usual sedatory effect seen in other conditions, the serotonin system is very complex and plays a large part in controlling other monoamine systems via heteromers etc.... Quetiapine with SRIs are given in part to level the catecholamine hyperdrive seen in bipolar episodes and as well if needed to mediate the mentioned SRI effect when the SRI is used for the depressive phases of the condition. It's vaguely and in short "keeping a lid on things".

Now do they interact? - directly, often not. Some SRIs carry binding affinities to some serotonin receptors as secondary effects to their primary SRI action, but many do not. We should note here quetiapine is effective in blocking serotonin receptors (it's potent) and its action is of competitive nature (concentration dependent) so if indeed we'd see a situation where quetiapine is prescribed with an SRI that also activates serotonin receptors as an exogenous ligand itself it would more likely than not be inhibited at the sites by quetiapine, rendering these specific effects mute. This is regarding pharmacodynamics.