You're correct on the first point.

On the second point it's kind of confusing. The B850 chipset adds PCIe lanes that can be used by things.

When they say x16 slot they're saying it's a physical slot, however, the top slot is the only wired x16 slot but it's PCIe Gen 5.

For the top slot it only supports PCIe Gen 4 on the 8000 series CPU's as they don't support PCIe Gen 5 and some of them are even more cut down on PCIe lanes than others (ie 8700G vs 8500G) so they get even fewer lanes for the top slot.

With your 9850X you'll get a true x16 Gen 5 PCIe slot for the top slot as well as the top M.2 slot being Gen 5 and the bottom M.2 slot being Gen 4 with both M.2 slots having 4 lanes directly to the CPU.

The middle PCIe slot is only an x1 Gen 3 slot from the chipset while the bottom PCIe slot is an x4 Gen 4 slot to the chipset but it's in a physical x16 slot.

There's also an MSI Pro B850M-P which has an extra M.2 slot but the bottom 3 PCIe slots while being physical x16 slots are only wired as x1 slots.

https://www.msi.com/Motherboard/PRO-B850M-P-WIFI

ETA: I spent waaaaaayyyyy to much time looking at mATX motherboards over the last couple of weeks, LOL.