Most Symmes' world have large openings in the shell (often at the poles) which allow the atmospheres, hydrospheres and biospheres of the outside surface and inside volume to mix.
A Symmes' World starts out as a hollowed out asteroid or the shell of a supramundane planet but as the asteroid's mass is mined out or the underbody's mass converted into heavier matter it is applied to the shell, along with any Dynamic Compression Members that may be needed to brace it.
In the case of engineered jovians, when the shell becomes thick enough (10,000km is adequate) it generates enough of a gravity well to hold on to a biosphere without help from the underbody. Thinner shells may be use for smaller bodies but a Worldhouse roof, Dyson worldtree or Yggdrasil bush may be need to retain the biosphere. However as there is no net gravity inside the shell when the underbody is removed only a freefall ecology can be set up inside. A shell of 100,000km in diameter or less may be filled with a breathable gas mixture and large openings in the shell (most often at the poles) called "Symmes' holes" allow the atmospheres, hydrospheres and biospheres of the outside surface and inside volume to mix as one. A central luminaire may act as a sun and is kept centered by polyfullerene cables running to the shell. The cables can also act as anchoring for cities and ecologies along their length. Other lighting systems may also be used; mirrors over the Symmes' holes or sunlamps hung from the shell.
The weather system of a Symmes' World is made more complex by the different day/night cycles of the outside and inside. On the outside one half is always lit by the local sun while on the inside all of the volume is lit by the luminaire for half the time. Waste heat (heavy industry use may be timed to use the surplus energy made available with the cyclic luminaire shutdown) and the remote locations of the connecting Symmes' holes complicate airflow patterns.
Because a Symmes' World uses mass to generate gravity effects when more 'matter efficient' means can be used they are not popular in most polities. However the MPA has built some for artistic reasons and the Red Star 'M'pire makes such shells do 'double duty' by building them of computronium.