The believer lives within Allah’s commands. Not everything a person desires is permissible; lawful and unlawful choices, others’ rights, and accountability set boundaries. Even great comfort in this world is temporary when compared with the lasting blessings of the Hereafter.
Calling this world a prison does not imply that every believer must be poor or unhappy. Wealth, health, and a comfortable home do not cancel the comparison. The point is the world’s limited duration, freedoms, and pleasures relative to what follows.
Similarly, describing the world as a paradise for the unbeliever is not a claim that every unbeliever enjoys constant happiness; the report makes a comparison with the Hereafter rather than a list of present circumstances.