Since Shopware 6.5, you can find the Pickware menu items Warehousing and Purchasing in the Shopware Admin under Extensions by default. If the Warehousing menu item is missing from the main menu or you can't find it, check there first.
If Warehousing is located under Extensions for you, click paths start there accordingly, for example Extensions → Warehousing → Stock overview instead of Warehousing → Stock overview.
Bringing Warehousing and Purchasing into the main menu
You can also display the Pickware features directly in the main menu of the Shopware Admin:
Open Settings → Extensions → Pickware ERP.
In the section Darstellung im Shopware Menü (Display in the Shopware menu), enable the option Pickware Funktionen direkt im Menü statt unter Erweiterungen anzeigen (empfohlen) (Show Pickware features directly in the menu instead of under Extensions (recommended)).
Save and reload the Shopware Admin. The menu only changes after reloading.
Afterwards, Warehousing and Purchasing are available again as separate menu items in the main menu.
You can read more about the changes with Shopware 6.5 in What do I need to consider when updating to Shopware 6.5?.