Flows triggered by other flows can be a little confusing. They need to be created in reverse order, but you should think forward in the process.
For App A, if you wish to update all related items in App B: - set up a Manual Flow with App B as the working app, triggered by App A - add an Action to this flow to update the item - then Create a Flow in App A to trigger this flow when relevant item fields are updated
See the Trigger Flow on Current and Trigger Flow on Related action steps for more info.
Example:
First, set up a Manual Flow on the Tickets App, and triggered by the Timesheets App, to update the Last Activity Date field on ticket
Be sure to select where the app will be triggered, default is set to the Current App and will not run correctly when triggered from another app.
Set up the flow that will trigger this Manual Flow on the Timesheets App.
Select Trigger Flow on Related to trigger a manual flow created in another app
Select Trigger Flow to trigger a manual flow created in the same app.