For on-disk storage (in memory largely is different, although the on-disk format is used in a few places too), variable length datums are stored as 'varlenas'.

For on-disk storage (in memory largely is different, although the on-disk format is used in a few places too), variable length datums are stored as 'varlenas'. Basically there's a 'varlena header' indicating the length of in-line stored data (1 byte for short contents up to 126 bytes long, 4 byte header after that). For data longer than 126 bytes the data can be compressed inline. Besides being storing the actual data, compressed or uncompressed, inline, the inline data may also just be a pointer to data stored outside the main table ("TOAST"). Where data again can be compressed or uncompressed depending on settings and compressability. We do utilize pointers-into-the-buffer-pool in some places, to point to variable length columns in the buffer pool (obviously only while we have pinned the respective page(s)). That's never used for by-value data types. Hopes that answers your question?