If you are using native L-mount lenses, I believe the camera should automatically detect when an APS-C lens is attached and switch into Super35/APS-C crop mode. Panasonic hasn't released any L-mount APS-C lenses yet, but Leica does have some available (these lenses are usually designated TL, whereas Leica's full frame L-mount lenses are designated SL). It's possible this automatic detection of the lens coverage area will also work with EF mount APS-C lenses and the Sigma EF to L-mount adapter, but I have not actually tried this myself (Sigma's page for the MC-21 adapter seems to suggest that this will work, at least with their own APS-C EF mount lenses: https://www.sigma-global.com/en/lenses/cas/product/accessories/mount-converter/#mc-21).
If you are using a lens that does not have an electrical connection to the camera (such as manual lenses with an adapter), then when you turn the camera on you should be prompted to set the focal length for the lens. When you go in to edit the focal length information, you will see that it is also possible to set the lens image circle area to either Full or APS-C coverage. You can create lens information configurations for up to 12 lenses that are saved in the camera. Note that it is possible to turn off this automatic prompting to set the focal length for a manual lens, in which case you will have to use the Lens Information item in the menus to set this information for the lens.
For native, full-frame lenses, it does not seem to be possible to explicitly enable APS-C crop mode in photo mode (at least I could not find a way to do so). You can, however, enable the Extra Tele Conversion (ETC) function, which will give you about a 1.4x crop (close to APS-C) when shooting M (medium resolution) photos. If you switch to S for the photo size, then ETC mode allows for a 2x crop. Note that there are actually two different options in the ETC menu, Zoom and Tele Conversion. The Tele Conversion option does a fixed crop (1.4x or 2x depending on the photo size), whereas the Zoom option allows you to adjust the crop amount (from 1x to 1.4x for M-sized photos, or from 1x to 2x for S-sized photos). To access this ETC zoom function after setting that option in the menu, you either need to assign it to a function Fn button or enable the touch screen menu tabs (in which case the slide out touch menu will then include the zoom controls).
