A 36M is like having two 18Ms in one module, providing two banks of 18 cues each. In contrast, a 72M is similar to having four 18Ms in a single module, thereby providing four banks of 18 cues each. You can assign each bank to a specific channel; for example, bank A assigned to channel 01 and bank B assigned to channel 02. Using the 18R or 18R2, you select a channel and fire accordingly.
To change the channel for each respective bank, you can press the CHANGE BANK button on the 36M or 72M to toggle to the different banks. Once on the desired bank, press and hold the CH+ or CH- momentarily to change the channel. There is a "bump protection" designed to protect against accidental channel changes, so a momentary press is required.
For more information on banks and channeling on the 36M or 72M, see What is a bank, and how does the 36M/72M function with the 18R and 18R2?