Look like cardboard (could be leather, possibly more than one layer) soles were hole-punched then several stitches were worked in each hole. These are where the yarn is worked into to create the upper. Will look for similar to help with pattern.
Free Pattern - #crochet slippers
crochet socks to wear around the house
easy peasy knitted slippers!! <3
Basic Clog Slipper pattern - crochet
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