I have installed Oracle RAC 19c on RHEL 8.1, 8.2, 8.4, 8.8, also on 18c, 21c, also on Oracle Linux 7.9, 8.5.
I always choose the CDB configuration and install in local UNDO mode, creating only PDB$SEED, PDB will be created later.
I have always wondered why PDB$SEED datafiles are placed under the directory named GUID in CDB$ROOT?
An example would be as follows.
Under +DATA/ORCL/<GUID>/DATAFILE/ of CDB$ROOT, the system generated files of SYSTEM/SYSAUX/UNDOTBS1 of PDB$SEED are stored under +DATA/ORCL/<GUID>/TEMPFILE/ of PDB$SEED, PDB$SEED's TEMPFILE system generated files are stored under +DATA/ORCL/<PDB$SEED's GUID>/TEMPFILE/.
This occurs whether or not Oracle Managed Files is selected.
This does not happen in a single instance. If CDB$ROOT datafiles are stored under /u01/app/oracle/oradata/ORCL/, PDB$SEED datafiles are stored under /u01/app/oracle/oradata/ORCL/pdbseed/. Both datafiles are separated.
However, in RAC, the alias file of PDB$SEED generated under +DATA/ORCL/pdbseed/ refers to the system generated file under +DATA/ORCL/<GUID of CDB$ROOT>/DATAFILE/.