Is there an English word/term/phrase for when you are working on something while using it at the same time?
Solution 1:
As suggested, bootstrapping is a good term for building your tools with the tools you've built. Turning it around, using the tools you built is sometimes called dogfooding
Solution 2:
Welcome, Lakesha!
I've heard it called changing the tire while the car is moving and putting the wing back while the plane is flying.
Solution 3:
It sounds like an extension of concurrency which relates to parallel execution. Here you are developing a system concurrently with the system's usage. This isn't precisely what you described, but seems to come close.