I claim the ornamental design for a wearable device core and holder, substantially as shown and described.
FIG. 1 is a top-front perspective view of a wearable device core situated within a holder with a band connected to the holder;
FIG. 2 is another top-front perspective view of a wearable device core situated within a holder with a band connected to the holder;
FIG. 3 is a front elevational view of a wearable device core situated within a holder with a band connected to the holder;
FIG. 4 is another front elevational view of a wearable device core situated within a holder;
FIG. 5 is a top elevational view of a wearable device core situated within a holder;
FIG. 6 is a bottom elevational view of a wearable device core situated within a holder;
FIG. 7 is a right elevational view of a wearable device core situated within a holder;
FIG. 8 is a left elevational view of a wearable device core situated within a holder;
FIG. 9 is a front-right perspective view of a wearable device situated within a holder; and,
FIG. 10 is top-left perspective view of a wearable device core situated within a holder.
The broken lines shown in FIGS. 1-10 illustrate portions of the wearable device and holder that form no part of the claimed design. the additional broken lines that show the band in FIGS. 1-3 illustrate environment and form no part of the claimed design.