Zotero excels when used on a computer or laptop. Some Zotero features are not accessible via a cell phone.
To use Zotero with iOS, download the Zotero App.
Find additional third party Zotero apps for cell phones here.
Where does Zotero place citations from my cell phone?
Any entries that you create on a cell phone are added to your general "My Library" collection folder. You don't get to choose into which folder something gets placed. But this is easy enough to fix: Use a saved search of recent additions or create a folder of recent additions.
Create a Recent Addition Folder
Create a folder that contains all recently added items (recently added: 14 days). To create such a folder, just follow tip number 8.
Use a Saved Search
Often you'll add new items without worrying about tagging or putting them in collections. To set up a saved search for new items, click Advanced search, select “Dated Added” > “is in the last” > X “days/months” and fill in the desired period; then save the search. This gives you a dynamically updated overview of your latest additions, so that you can go back to them and do the categorization and tagging work when it suits you. It is recommended that you put checks in the boxes below. Also consider include date, date added, and date modified to ensure that everything is "caught".
Once you are back at your computer, you just drag+drop the entries that you saved into the desired folders.