Being quite the amateur and just experimenting - i found that not only does the order of the layer matter but also what you apply to each layer. I was trying out layer modes to see what they do. I made a couple rings used a radial gradient to get this:



Then made another rings layer and filled with the pattern Unsharp Swirl. From here it got interesting.
These are all with the same two layers with different layer modes or different layer order:
Two different overlay results

the Multiply trial i had to lighten just a tad. but it made me think of the tv series Stranger Things
The thing i noticed here is that this pattern needs to stay relatively small or it gets blurry

Was just playing and trying things. I.m sure there are plenty of layer mode posts.
i had a lot more examples but i liked that last one i think the best out of about two dozen

leftyleo, you have got there very interesting colours and textures.
Yes, experimentation and observation is much the base in the learning line.
Thanks for sharing them.

I have learned with many applications over the years of learning new software and even coding - that you cannot be afraid of making mistakes. save a copy or make a new layer whatever you need to do before trying that next experiment. I am also working through a class on udemy (am i allowed to say that?) and i made a mistake that worked out better than the effect i was going for.

Thank you for the encouragement

The layer and its blend mode are applied over the composition of all the layers below it in the stack.

If you use layer groups, things are a bit different, groups act as parentheses is algebra, Gimp computes the result of the group by compositing the layers within the group, and then uses that instead of the group at the level above.

Also with most blend mode (except the top group: Normal, Erase...) the added layer doesn't change the opacity of the result.

I think i understand . Now I need to go back and keep track of where the layer is and what mode i apply.

You said it applies to layers below it? i think i got results with it below? but the mode applies to the belows as you say then the top was just an above layer as normal.

Interesting. Thank you -- back to play some more