Here's an at-a-glance selection of some ways to refer to different kinds of stored data in basic widgets, using attributes:
Buttons: .value (boolean true/false)
Fields: .text (plain text) .value (rich text) .data (LOVE data) or .images (images only)
(it's a little visually chaotic but an interactive example of the difference between these four can be found here)
Canvases: .copy[] and .paste[]
Sliders: .value (the number it's currently set to)
Grids: .value (for the whole table), .rowvalue and .cellvalue
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And to refer to a .value attribute of a widget on the same card:
widgetname.value
Or to refer to a .value of a widget on another card:
othercardname.widgets.widgetname.value
And if things have less script-friendly names you can refer to them with square brackets:
deck.cards["Silly Card Name"].widgets["I'm a Widget"].value
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I hope this helps point you in the right direction!