Scheme of extraction procedure for CBP example outside CH (upper row of Fig. 1). The upper panel shows the procedure for the SDO/AIA 193 Å image of the CBP, consisting of smoothing, longitudinal and latitudinal edge detection (ED) and background subtraction (BS), and finally stacking step. The bottom panel shows the same procedure for the ALMA Band 6 image of the same CBP, but with no smoothing required. The green contour outlines the edge of a CBP of interest. The same procedure for a CBP inside of CH, given in the bottom row of Fig. 1, is given in Fig. A.1. All cutout images have the same spatial scale as the corresponding cutout images in Fig. 1, but are centred on the brightest CBP feature.