worship one's ancestors, entomb
祔 centers on ritual connection to ancestors: worshiping them and, by extension, entombing them together in a shared ancestral shrine or grave.
祔 combines 示 (altar, spirit) with 付, which likely contributes the sound and may suggest attaching or adding, reflecting the idea of joining ancestors in worship or burial.
The altar radical 示 stands beside 付, which looks like a person attaching something. Picture a person attaching an offering to the altar to worship ancestors and entomb them together.
For フ, imagine a family saying 'phew' with relief after properly entombing their ancestors: phew -> フ, and the altar 示 shows the ritual is complete.