The bubbles that nucleated during slow-roll inflation can be supercritical, i.e., their radii are larger than the Hubble horizon of de Sitter spacetime inside the bubble (an inflating baby universe inside it), and thus naturally develop to the supermassive primordial black holes (SMPBHs) with a multipeak mass function. In this paper, we further investigate relevant phenomenology. After slow-roll inflation ended, the bubbles may be not only supercritical, but also subcritical. It is shown that it seems unlikely for the subcritical bubbles to collapse to SMPBHs. Theoretically, however, before they collapsed such bubbles might have a probability of up tunneling to the supercritical ones and thus contribute to SMPBHs. We present a mechanism for the origin of initial clustering of SMPBHs, which can significantly magnify the merger rate of SMPBH binaries and show the possibility that the merging of such SMPBH binaries explains recent NANOGrav signals.