G protein-coupled receptors (GPCRs) form the largest superfamily of cell membrane receptors in eukaryotes and are prime targets for therapeutic development. Among them, adhesion GPCRs (aGPCRs) form the second largest family, characterized by their large, multidomain extracellular regions (ECRs) that mediate cell adhesion and play roles in transmitting extracellular signals to the inside of the cell. Despite their significance, full-length structures of aGPCRs and the molecular mechanisms underlying their downstream signaling remain poorly understood. This thesis focuses on Latrophilin3/ADGRL3, a prototypic aGPCR, and aims to characterize ECR-mediated signal transduction at the molecular level. We employed single particle cryo-EM analysis and single-molecule FRET experiments to reveal the relative orientation and dynamics between the ECR and seven-pass transmembrane (7TM) region of the ADGRL3 holoreceptor. We also used phage-display technology to develop synthetic antigen binders (sABs) to specifically target ADGRL3. sABs targeting the GAIN domain, as well as cancer-associated mutations at the GAIN-7TM interface, alter holoreceptor conformations and modulate downstream signaling, revealing functional coupling between the ECR and 7TM. Among generated binders we also characterized sAB LK30 as isoform- and ligand-specific modulator of ADGRL3 function. Altogether, the work presented here demonstrates that ADGRL3 function can be modulated through shifts in the relative orientation of the ECR to the 7TM, and this may be true for all aGPCRs. Moreover, we validated a panel of sABs which will serve as valuable tools for future studies of ADGRL3 and lay groundwork for the development of therapeutics targeting this intriguing family of receptors.