Virtually all MySQL indexes are BTrees.
All of MyISAM's indexes are separate from the data, hence not clustered.
InnoDB's PRIMARY KEY is "clustered" with the data. Hence a "range scan" on the PK is very efficient.
InnoDB's secondary keys include the PK. Hence, a secondary key lookup requires a second lookup to get to the data.
Given a SELECT, we can discuss what index to make that would optimize the query. The answer almost never mention primary, clustering, secondary or BTree. It will list the field(s) that should be in the INDEX, and which order they should be in.
Separate from that question, InnoDB tables should always have a PRIMARY KEY. This should be the 'natural' key or an artificial AUTO_INCREMENT. Again, knowing the SELECT(s) is necessary to be more specific.
To create the optimal index(es), all the important SELECTs, DELETEs, and UPDATEs, including JOINs, need to be discussed simultaneously.