They send specs, usually in the form of an excel spreadsheet with a line drawing (done in Adobe illustrator or in pen). This type of drawing is known as Flat Fashion Drawing or flats.
This will usually be accompanied by fabric swatches, perhaps a spec for a print (in colour also illustrator, but sent as a pdf), perhpas the designs coloured up but still in flats format.
Everywhere I’ve ever worked that has a clothing dept (mostly athletic branded) has specified their work in this way.