Search notes:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Microsoft SQL Server\Instance Names

The subkeys of HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Microsoft SQL Server\Instance Names maintain a mapping of instance names to instance IDs.
For example, the subkey SQL might contain the mapping MSSQLSERVER = MSSQL14.MSSQLSERVER:

Subkeys

Some (all?) possible subkeys are
Subkey Realm
SQL
OLAP
RS Reporting Services

See also

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Microsoft SQL Server\instance name

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