There are two (or more) "families" of Neo carts. What I mean by that is that the 150-in-1 is the latest "model" of one manufacturers board and the 138-in-1 is the latest "model" of another manufacturers board. I am not sure exactly how the 138-in-1 functions as I don't own one. Perhaps someone who has experience with the cart can chime in?
The 150-in-1 basically is set up where when you power up, it reads the "menu" as a MVS cart and treats it as such... I stick it in to cart slot 1 so when I first power up it loads to the menu rather than having it go to the other carts first then to the menu (where it stays). Once at the menu you select a game and the system self-reboots... when the system comes up, instead of the menu "cart" in slot 1 it's the game you selected and until you go back to the menu, the system functions like a typical 4-slot MVS setup with the multicart being recognized as the game you selected....attracts all work, etc. You can play any of the 4 slots... but of course you can only go back to the menu by holding the P1 START button for a few seconds while you are back at the game you selected prior. When the menu is loaded, it essentially reboots with the menu software loaded as cart 1 and you can then select a different game.
I've found a glitch or two requiring a cart be moved to work right (for example, with the 150-in-1 installed King of Monsters 2 will only work on slot 2 and 4... not 3 but with 150-in-1 out it works on all 4 slots...). But that is it...