Comment by ClikeX

Depends. With all the web agencies I've made, the only code that belonged to customers was the actual website part. Any of the "jigs" that we made for our workflow was not part of that.

And contractually, any code I made was my employer's if I made it during office hours. Some even made a claim for code I would've written that during my employ that would be "competitive". Luckily, there was a massive difference in what I would do in my own time versus what they did.