Unlike ‘add’ chords that are built by adding a single note to a triad, EXTENDED chords are built by adding one or more notes to a 7th chord. The most common 7ths to extend are MAJOR, minor and Dominant.
The following graphic showing the building process for EXTENDED MAJOR chords will do more to cement your understanding of how they’re constructed than any 1,000 words I could write. Up first, MAJOR triads, 7ths, 9ths, 11ths and 13ths. The notes that occur in the 7th chord are considered “tones”, the notes that occur in the next octave are considered “tensions”.
Note how everything is built first from the TRIAD and then from the MAJOR 7th.
The same process works for all other types of 7ths as well. MAJOR, minor, Dominant and Dominant minor 9, 11 and 13 chords are all fairly common in jazz and work basically the same way. The only real difference is you would start with the appropriate 7th and build from there.