Yes, Linux marches to the beat of a different... kazoo.
When you install Ubuntu, and most other Linux OS, you are given the choice of where multiple directories go. What goes in them is defined in the Filesystem Hierarchy Standard, including (but not limited to) locations such as
/boot/efi (for UEFI installs only)
Each can be assigned to a different physical device, and https://unix.stackexchange.com/questions/5489/how-to-safely-move-tmp-to-a-different-volume illustrates how to do it post-install for /tmp . It's far, far better to do it during the install process; thank you for thinking ahead.
I welcome commentary from those more experienced on the subject of which directories should go in the 32 GB drive and which should not, especially with the changes made by Snaps and other packaging methods. I think this would be a reasonable division:
The soldered-in 32 GB eMMC drive:
The externally mounted SD card
It's too darned easy to eject an SD card by accident on most laptops. Do that, and likely you have a system crash.
Either put some kind of Molly-Guard over the SD card and its ejection mechanism, and/or consider supplementing the 32 GB internal drive with something larger that doesn't use the SD slot.
I did that in one of my laptops, and use the SD card slot for backup. It had a removable eMMC card slot, so I got a 512GB eMMC card, which works very well. Without knowing your laptop make and model, can't see if that would work for you.
Also, SD card file access is generally slower than a eMMC drive would be, another reason why I advocate replacing your internal drive. Make sure not to order an M.2 or M2 drive if you choose that solution, for those drives will not work in an eMMC slot.
Lastly, with any solid state device, performance will decline if you come close to filling it. A credible technical website found that “minimum performance improves substantially once you hit 25% spare area for these (consumer) drives.” Their final recommendation was that you should “plan on using only about 75% of (your drive’s) capacity if you want a good balance between performance consistency and capacity.”