All memory configurations with your soldered onboard 8GB memory will be given you dual channel support.

@Chino de Oro​ yes that's from the link that I posted which also includes the NOTE. 

Let me put it this way. If I buy a single 32GB I'll have 40GB with the 8GB onboard. The first 8GB will be dual channel and the rest not. Will the rest work seamlessly but just slower? Or what?

According to the JESD79-5 DDR5 SDRAM standard, with DDR5, each DIMM will have two channels. Each of these channels will be 40-bits wide: 32 data bits with eight ECC bits. While the data width is the same (64-bits total) having two smaller independent channels improves memory access efficiency.  So not only do you get the benefit of the speed bump with DDR5, the benefit of that higher MT/s is amplified by greater efficiency.

If your Dell system board was designed with memory architecture based this standard JEDEC principle, with the maximum upgrade, all your 40 GB will be operate at dual channel.

But that would directly contradict the note on the page we both quoted from and I included in my post.

<<Dual-channel support for mismatched memory configurations is limited to the size of the smallest memory module installed..>>

You are missing the big If from my last statement.  In your system, the memory configurations are supported as listing on previous comment and on the system specs.  Your concern does not make any different of how it will work.  Your only option is how much memory you want.  A total of 40 GB will certainly be better performance than a 16 GB, no matter how you look at it.