I use evernote, it not only allows you to record your ideas but you can also sync or share these ideas. For more details you can read its detailed review here: https://iosappers.com/app/evernote-review/
It is a free app
After using quite some note taking apps, I am using Notes for 8 months now and I like its simplicity, sync speed and the search on attachments. The text search function is fast, I feel no need for tags.