You carry that decent camera around with you because you want decent photos. Every so often, then, you need to get some of those unprocessed raw files on to your phone. How do you do it?
It's a well worn trope and the argument is easily won: for nearly everything, shoot raw files. They are your digital negative and give your oodles of latitude to process your images to a desired envisioning. Which also reduces to this:
Shoot for the widest exposure and then process the heck out of it
That's not to say JPEGs are redundant. They aren't. Their benefits are accessibility and file size. If you have a raw file from a Nikon Z7 then a client is unlikely to be able to view it, but send them a JPEG and they are happy. Perhaps more importantly as a shooter, if you need fast write times and a large buffer (such as for sports), then JPEG is the way to go. For everything else there is the malleability of raw.
Unless of course you happen to want to process you raw file on a smartphone. If that's the case, then camera manufacturers don't want you to do it. For example, Sony's PlayMemories and Fuji's CameraRemote only allow you to transfer JPEGs which leaves you using in-camera raw processors of which some are better than others. Just when you wanted to take advantage of the computational prowess of your smartphone you're, well, not allowed to. That's not something to pass up either with the extensive capabilities of the likes of Snapseed and it's more recent ability to process raw files.
At a time when camera manufacturers should be embracing open platforms and computational photography, we see them being at best mildly obstructive, at worst a closed platform. So what options are their for pulling raw files off your camera? The obvious one is to plug it in to a laptop, which gives you the greatest amount of flexibility in image processing but kinda defeats the object of using a smartphone in the first place!
Thinking slightly more laterally, a WiFi SDCard (such as the Toshiba FlashAir) provides full access to the card contents allowing you to transfer the full raw files back to your smartphone. That's a good option although you need to be aware of the power drain your camera battery, more limited support for faster card speeds, and the higher cost.
A related option is to use a WiFi hotspot card reader such as the Kingston Mobilelite Wireless range (like the G3). Take a card reader with you, plug it in to the hotspot and then access via the associated app. It works well and often doubles up as an emergency power bank. One useful tip is to ensure it's not a multiple card reader as some mobile devices struggle to work correctly when presented with multiple drives in a single reader. Also try get the fastest card reader available to maximize your transfer speeds through the entire processing chain.
If WiFi is not an option, then directly connecting your memory card to your phone might work for you. On devices with a microUSB connection, Android may support the use of an On-the-Go (OTG) cable which will mount the drive via a card reader. Support is much better for USBC type connections where a similar connector is available. On iOS it's simpler (and pricier) with a Lightning to SD Card Camera Reader cable.
A word of warning on any of the methods that require you to plug your memory card in to another device. Your camera has well tested firmware inside it, where formatting and reading and writing files to a memory card is highly reliable. It is uncommon for cards to become corrupted and usually only when there has been excessive wear and tear or they have been damaged when out of the camera. The same is not true of less than well tested firmware, often found in peripheral products with short product life cycles. Just be aware that the potential for file loss is higher, although not necessarily full card loss. Related to this, care is required when unplugging a card before ejecting it from a device. Perhaps the best starting point is taking some test shots and then trialing the workflow before you need to use it. As ever, make sure everything is fully charged, as flat or near-flat devices can lead to all sorts of problems. You have been warned!
Once you've copied the required images off your memory card, you then need to ingest them in to your raw processor of choice. There are apps, such as PhotoMate, that will read your raw files directly, however for many people Snapseed or Adobe Photoshop Lightroom will be their first choice. Raw support (on Android) means a DNG, so the next port of call is a raw file convertor. For my Nikon D700, I found the free raw2dng worked impeccably for converting my NEF files for subsequent editing. A list of supported cameras can be viewed at the libraw website and whilst fairly comprehensive, support for the latest models lags behind commercial products (e.g. no Nikon Z 6 or Z 7). Likewise, whilst Fuji images can be processed, the developer of raw2dng notes that X-Trans image conversion is not as good as other conversion software.
Once you've started the app, simply select the raw file you want to process, then choose the conversion of choice: JPEG, DNG or TIFF.
If you convert an existing DNG (for example, that was shot on your smartphone) then it will convert it to a lossless compressed DNG. Great for saving space. Most usefully, if your camera supports a WiFI connection then raw2dng may be able to connect to your camera if it uses the PTP-IP protocol. The developer has tested it with some Sony and Canon cameras, but support may vary. I tried connecting my Sony RX100M2 and at the first attempt it didn't work, due in no small part to Sony's appalling documentation!
In the camera settings you will need to set the USB connection to auto and then go to Access point settings and connect the camera to your house WiFi. Once done, install PlayMemories on to your PC, connect the camera via a USB cable and, when the camera settings dialog appears, go in to WiFi import settings to complete the setup. raw2dng will then be able to connect directly to the camera and access the raw files. Bear in mind that the connection is made via your house WiFi, not directly to the camera so it won't work where you aren't able to connect both the camera and phone to an access point.
Taking advantage of the processing power of your smartphone and editing images when you are on the move shouldn't be a chore, yet camera manufacturers are making it more difficult than it need be. We are starting to see greater interoperability through the convergence of technologies such that full blown raw processing is now possible. It shouldn't be sniffed at because there are times when you want to post, print, or deliver a file to a client when you're on the move. Do you already edit raw files on your smartphone? Have you got a tried and tested way of moving data around? Are there any apps which you'd recommend as part of the processing chain?