The alto ‘C’ and tenor ‘C’ clefs are so called because the clefs identify the middle ‘C’ line - where that little indentation or crinkle in the clef is.

The alto ‘C’  clef identifies the  middle 3rd line as the middle ‘C’ line. This is on a lower line in the staff than the tenor clef because altos sing higher than tenors.

The tenor ‘C’  clef identifies the  4th line up or second line down as the middle ‘C’ line. This is on a higher line in the staff than the alto clef because tenors sing lower than altos.

The tenor 8 clef is only used in those occasional questions about recasting scores from open to closed score or vice versa.  It’s a convenient way of writing music for tenors in the treble clef on the understanding that they sing the notes an octave lower