A measure stores a formula, not a number.
A table with no data in it
All 219 measures live in one table, _Measures. It holds no rows and has no relationships. It exists so every measure has one home, and the Fields pane shows them in one place instead of scattered across fact tables.
A measure doesn't store a result. It stores a formula, and Power BI works it out for each cell of a visual, using whatever filters apply to that cell. That set of filters is the filter context.
Total tonnes sits in a matrix: commodities down the side, years across the top, with totals. How many times is it calculated?
This cell's filter context
Click any number, including a total. A toy table with illustrative values.
Every measure has a register row
A number on a tile is only useful if people know what it counts. So every measure shown on a tile has a row in the measure basis register, the DimMeasureBasis table, with six fields:
The short explanation behind each tile's information button is a measure too. It reads its text from the register, so the wording lives in one place.