At the end of the day, you need to do a good job and feel invested. There are lectures where I’ve changed very little from what colleagues have given me—eg, methods and stats—and others where I look at the syllabus and use some readings but mostly put it all together myself.
Case by case.
As a rule, if materials are available for you that are better than you can make in the time available, use them, at least for inspiration. Don’t listen to these self obsessed ding dongs.
You’ll find your way.
And by the way, having taught both high school and college for many years, this Is the first time I’ve seen people being possessive about their stuff. Lame as hell and also stupid.