Primary Key and Indexes

Hi, I really enjoy the course “An Introductory guide to SQL”. I would say it is the best SQL course I have ever seen. Thank you a lot for your hard work.
I am quite lost in the section of Primary Key and Indexes, could you please explain them again step by step how the data (here the table will be stored and how to index for both cases clustered and non-clustered). I tried to read and look at the graphs but still could not understand how it works. Thank you a lot!

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The clustered index is the one that links each row to its physical location on the hard disk. Leaf nodes, in this case, include pages that contain the actual data. On the other hand, the non-clustered index stores a pointer to data stored elsewhere on the disk in leaf nodes. The MyISAM database engine, or ISAM (Indexed Sequential Access Method), is an indexing algorithm developed by IBM that allows retrieving information from large sets of data in a fast way, especially when using a heap table with no order.

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