See Deploying charmed operators offline.
The exemplar snap for Juju users is charmed Operator Tools but the guidance provided here can be applied to any snap. See Installing snaps offline.
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Many docs for juju are written in discourse then published here: https://juju.is/docs/juju.
The links provided are purposefully archived. The official location for the information you’re looking for starts here: https://juju.is/docs/juju/working-offline