A QR Code consists of a sequence of segments. Each segment has a mode - numeric, alphanumeric, kanji, or byte. It is possible to shift between encoding regimes by ending a segment and beginning a new segment with a different mode. https://www.nayuki.io/page/optimal-text-segmentation-for-qr-...

Some 1D barcodes have inline shift symbols like you said for Data Matrix, though. e.g. https://en.wikipedia.org/wiki/Code_128