HTB is the hierarchical token bucket shaper that is used by simple.qos and simplest.qos to restrict the bandwidth (see https://linux.die.net/man/8/tc-htb for a bit more information). We combine HTB to restrict the bandwidth and fq_codel to divide that restricted bandwidth fair between the flows. Cake can perform both objectives and hence does not need to be used with HTB. Simple.qos uses 3 priority tiers/bands and is roughly equivalent with layer_cake.qos, while simplest.qos only uses one priority tier and is roughly equivalent with piece_of_cake.qos.
Hope that helps…