There are many touch points of a brand, for the digital presence of the brand, these touchpoints come in the form of elements viewed on screens. But regardless of what any of these elements are since they all must be displayed on a screen, there are only so many dimensions that they can differ. The most important dimensions are color, typography, sizing, spacing, and alignment.
We as humans have clustered these screens based on these dimensions into what we call tone or vibe. Our brains detect the difference in color, typography, sizing, spacing, and alignment between the Crayola app and the American Express app and cluster them accordingly. We perceive the Crayola app as a fun app for kids, and we perceive the American Express app as a serious and important app.
As you design, you should reuse similar ideas about these different dimensions to keep the look and feel of your app congruent.