Would it matter if you used any one of these as a starting point for your own?Who's to say you didn't just silo a photo of your own hand and use that as your starting point?
So, instead, I copy the flame image from the link, paste into my graphic software and then use curve function to create the curves over the image in the link and create a flame vector art in this way for myself.
Is this way of using someone else's work as template to create my own icon violation of copyright?
Your flame icon, likewise, is something that has been created and re-created so many times and is so simple and iconic - and publicly available - that there is very little to indicate that one person or another created it.
Let's say I want to create a flame like this one and use it in my app: But, I'm not good enough to make it from scratch and make it look beautiful as in the link.
Demolish the delectably sugary X-Ray Speculoos creation or do DIY donuts.