I have few custom extension that . they appear in Store> Configuration section of admin
Each vendor has a entry in the side menu. I like to organize all those extension into one Section preferably creating a section at the bottom of the side bar under Advanced calling it Custom extension and moving all the extension into that location .
any suggestion has how to go about doing this ?
There's not a simple way of doing this. Certainly not without breaking your upgradability of those extensions when updates are available.
I think it is a good idea to have this extension somehow organized in their location rather having them mixed with magento core admin sections.
It is an interesting topic of conversation and one I can see either way. From a user point of view, it can make sense to integrate with the same section of core admin: additions to google analytics should go in the standard google analytics section from a user journey perspective. The spanner in the works is that many admin users are actually familiar with what functionality is core and what functionality is from an extension. From their perspective it makes sense to go to that extension's configuration section. That's why you'll end up finding a mix of solutions from extension vendors. There has been some pushback from the community of vendors having their own tab on the lefthand side, and certainly pushback of having it in the main navigation menu as it's self-inflated importance and can get in the way.
Now what we really need if for admin users to be able to customise the way their admin looks