@rfclitadim If you don't have trigger conditions for your Flow B—this flow will trigger any time an item is created or modified. You need to use trigger conditions to ensure that Flow B is only triggered when an item is modified and if the Assigned To column is not empty.
Without the trigger condition in place, Flow B will be triggered as soon as an item is created (which isn't your intention).
Flow C needs to also have a trigger condition to only trigger when the Status column value is Completed.
---
Compose a Trigger Condition
For testing purposes, create a brand new flow. Use the When an item or file is modified trigger.
Add a Compose action to store the Status value dynamic content.

To get the expression you need for the trigger condition—use a Filter Array action. Leave the From field blank. Use the two value fields.
In the first value field, insert the Status value dynamic content. Leave the operator as is equal to. In the second value field insert the Status you want to filter by. In your case it will be PENDING. Important: The text is case sensitive. It should match whatever you have in your SharePoint list. In my case I'm filtering by items with a Current status.

Click on the three dots and select Peek Code.

Highlight the text, between the double quote marks including the @ symbol. Copy this text to your clipboard.

Delete the Filter Array action.
In the flow trigger, click on the three dots and select Settings.

Scroll down to the bottom and click Add to add a new Trigger Condition.

Paste the expression from your clipboard into the Trigger Condition field.

Press Done and test it out. First, change the status of an item to anything BUT Pending. The flow should not trigger. Next, change the status of an item (or create a new item) with a Pending status. The flow should trigger.
You can confirm this by the output of the Compose action which will output the Status that triggered the flow.
... more to come...
Test this out for now.