There are a few different ways, you can for example use the sharing extension in combination with for example a “scratchpad” or “Miscellaneous” or “Inbox” project, like shown here: Sharing, Import, Export, and Printing
But you can also setup a workflow in the Shortcuts app for example, that you could run with a global shortcut, Siri, or the shortcuts menu item (or on iOS saved as an icon) that could even trigger the question what you’d want to capture, after which it would save this in the above “inbox” project as a new note, optionally even allowing for additional optional input like a date etc.
Plus, you could combine it with templates that can even take input, to make it even more powerful.