One of the key tools in GIMP is Layer Masks, which allow you to make selected parts of a layer transparent, so that lower layers can come through. This is a way to get the same kinds of effects you would get with physical transparency sheets or animation gels.

Links:

https://docs.gimp.org/2.10/en/

https://en.wikipedia.org/wiki/Wikipedia:Public_domain_image_resources

https://snappygoat.com/

https://snappygoat.com/s/?q=flowerscloseup#eda72d0e7eedc238eed8f7acffc515dba94ce64c,1,342

https://www.gimp.org/docs/userfaq.html#when-will-gimp-support-any-kind-of-non-destructive-editing-like-adjustment-layers-layer-filters-andor-full-blown-node-based-editing

https://www.ahuka.com/gimp/layer-masks/