Is the same Sailfish X OS image useable for Xperia X and XA2? [answered]

To expand a bit:

• the users-space side of things (most of the parts of Sailfish OS 3 that you see) are the same.
• BUT the kernel and drivers are radically different. You need to load a different firmware blob with drivers from Sony (Loire of X and Nile for XA2), Jolla has built a different kernel for each device, and each device needs a different libhybris hardware abstraction layer (what Jolla calls a "Baseport" in their blog posts) to use said layer. So the whole low-level layer is completely different and incompatible (welcome to the world of ARM embed hardware).
• In addition to the above, in the specific case of XA2 vs all the other past Jolla-supported hardware: XA2 happens to have also switched to a different new android application compatibility layer (Aliendalvik), with a new one that Jolla have rewritten to work using LXC.

I've once written an answer to a question about the internals of Sailfish. From that point of view:

• the GNU/Linux parts are shared between all smartphones supported by Jolla, and that's the reason you can still get Sailfish OS 3 update on the very first Jolla 1 phone.
• the Android-y parts at the bottom (Baseport) are different and absolutely smartphone-model dependent. Different hardware require diffent parts, and that's the thing you're actually paying Jolla for with the license (for them to develop an adaptation for a specific piece of hardware).