The sustainable use of recycled aggregates in concrete presents both opportunities and challenges for construction materials research. This study addresses a critical knowledge gap in understanding the interfacial transition zone (ITZ) characteristics when incorporating different types of recycled aggregates, exploring both crushed solid brick aggregate and recycled concrete aggregate treated with different methods before their use. Traditional concrete modeling approaches often simplify the interfacial transition zone as a uniform layer, overlooking its inherent spatial variability. The present research introduces a novel micromechanical multiscale modeling approach that comprehensively captures the complex heterogeneity of concrete, spanning observation scales from micrometers to centimeters. By integrating advanced characterization techniques including nanoindentation and SEM-EDX analysis, it is systematically investigated how different recycled aggregate types influence ITZ properties. Key findings reveal significant variations in ITZ thickness, porosity, and mechanical properties across different concrete regions, directly related to the specific type of recycled aggregate and its interaction with cement paste. The developed micromechanical model provides a sophisticated computational representation of central mechanical concrete properties, such as elastic stiffness and compressive strength, offering enhanced insights into the microstructural mechanisms governing concrete performance with recycled aggregates. This approach not only advances the fundamental understanding of concrete microstructure but also supports more informed decision-making in sustainable construction material design, potentially enabling more efficient use of recycled materials in concrete production.