How do 陷, 阱 appertain to their semantic component  (阝 as left radical)?
阜 are mounts of earth stacked on each other. Any character with semantic component 阜 (left ear) are usually related to earth, ground, and cliffs. Abundance of soil/earth is a borrowed meaning.
臽 literally was a trap. It's a man (人) dropping into a pit (a hole which looks sort of like a bowl/mortar 臼). Later on this character 臽 developed a different meaning by itself (it's an obsolete character now), and there were various semantics added to maintain the original meaning of 'trap', like 埳, and 陷. The cliff/mounds semantic version is the one used until today.
井 had various meanings, such as 'well', 'square fields' (which is what this character is based off), and 'pit'. A well is sort of an open hole placed in the middle of the square fields (丼).
Later on 井 was specifically used for well, while additional semantic components were added to clarify the 'pit' meaning in 阱 or 㘫. Currently trap is written as 陷阱, while 埳㘫 would be a variant form.
https://www.zdic.net/hans/%E9%99%B7
https://www.zdic.net/hans/%E9%98%B1