Solid-state spin defects such as nitrogen-vacancy (NV) centers in diamond, have emerged as powerful platforms for quantum sensing, enabling nanoscale measurements of magnetic fields, temperature, strain, and electric fields at both cryogenic temperatures as well as at ambient conditions. My research group focusses on utilizing these quantum sensors for non-invasive probing of isolated magnetic molecules on surfaces, which are difficult to access using other complementary techniques. These efforts naturally require the usage of near surface NV centers (< 10 nm) to gain desired magnetic sensitivity. However, such shallow NV sensors are also infamous for often lacking sufficient stability of their optical and spin properties. We take advantage of our UHV experimental conditions to understand the role of the diamond surface in defining these optical and spin properties for shallow NV sensors, as well as to better engineer the diamond surface with desired surface-chemistry. Another aspect of our current research involves two-dimensional mapping of both static and dynamic magnetization profile of synthetic antiferromagnets, as well as nanoscale devices, using scanning probe NV magnetometer. The key idea here is to identify their local nanoscale properties which are often obscured in any conventional transport measurements. After introducing the essential concepts of nanoscale quantum sensing and providing a broad overview of the current research scopes in my experimental group, I will also highlight on the current challenges and the opportunities in this vibrant field of research.