AAPL.O  maps  to AAPL US Equity (composite) or AAPL UW Equity (Nasdaq NSM)

That's straight common stock, but what about preferred stock?
HWM_p.A maps to EP051790@SAM2 Pf

or Unit trust?
PHGE_u.A  maps to PHGE/U UA Equity

or exchanges where Refinitiv don't use the exchange symbol?
FEDq.L maps to FEDS LI Equity

Futures the companies have different policies on when to introduce double digit (decase) future codes, and different policies on notation for pit electronic combines, am pm sessions 

The list goes on.

That's why Magtia remaps every exchange symbol every day before the exchange opens. (Disclaimer: I am employed by Magtia)