802.11ax/HE/Wi-Fi 6 introduced the concept of content channels in the HE preamble.
This is further developed into the 802.11be/EHT/Wi-Fi 7 standard. Content channels are used in the EHT MU frame format and therefore used in most of the EHT data frames sent between stations.
This blog article will explain content channel setup for DL EHT OFDMA frame on an 80 MHz data channel in Wi-Fi.
Read article: 
802.11be/EHT/Wi-Fi 7 Frame Format and the Use of Content Channels – Gjermund Raaen
It has been three years since my last deeply technical blog article. It is good to be back:).