And final discovery of the day:
When we build for xiao_ble_sense without sysbuild and without mcuboot, it knows where to put the application properly. You can notice in the build log that it considers the application will start on 0x27000 and due to that will have up to 788KB in size. I assume it gets that from partitions definitions on dts.
If you build with sysbuild and without mcuboot, it will not consider the partiioning information, place the application at 0x0000 and due to that consider up to 1MB in size. It won´t run because the adafruit bootloader will try to load application from 0x27000.
So I tested *with sysbuild and with mcuboot, so that I could use the pm_static.yml to tell zephyr where to place the application and it worked.
Tomorrow I’ll try the following: I want to use uf2 with sysbuild and without MCUBOOT on the this dual bootloader approach. I wonder if i can set the pm_static.yml without MCUBOOT to drive sysbuild to place stuff on the right place.