As 5G Resource Block (RB) concept is different than LTE as 5G RB is one-dimensional only (i.e. Frequency domain) only. 5G NR is 12 consecutive subcarriers in the frequency domain.
Unlike LTE, when RB is 12 subcarriers in frequency domain and 6-7 ofdma symbols in time domain based on the cyclic perfix.
The reason behind this one-dimensional allocation is the flexibility of the allocation of radio resources based on the OFDMA-symbol level only (mini-slot) that is more suitable for URLLC use cases.
3GPP TS 38.211 defined different concepts in the 5G NR resource grid, Physical and Common resource block. The Common Resource Block (CRB) numbers the resource blocks starting from a reference point (Point A) for fixed numerology. So What is Point A?
Point A Concept in 5G NR
Point A is a common reference point for all resource grids in the frequency domain, is the center of the subcarrier 0 of a common resource block 0 of the lowest resource grid, and can be outside of the carrier BW.
Each RG (resource grid) uses a specific 5G NR numerology and starts from a frequency offset from point A.