Inner coil wound RTD
This type of element is normally manufactured using platinum wire. Very small platinum wire (0.02 mm) is coiled and then slid into a small 2 holes ceramic insulator.
Larger extension leads are then spot welded to the ends of the platinum wire and cemented in place.
Some manufacturers backfill the bores of the insulator with ceramic powder, once the coil have been inserted.
This keeps the coils from moving and shorting against each other. The end opposite the extension leads is capped with ceramic cement also.