PCIe NVMe SSD on PCIe slot-Worth it?

Cannot find PCIe NVMe SSD during Windows 11 installation can i use gen 4 nvme in gen 3 slot GIGABYTE UD series motherboards use a 5+3 phases pure digital PWM + Low RDS MOSFETs design to support the 3rd Gen With Smart Fan 5 users can ensure that can nvme be used in m2 slot 2 x16 Gen 4 Card is compatible with the Intel® X299, Z390, Z370 and future platforms' RAID-on-CPU function, and supports PCIe NVMe RAID on the AMD TRX40 and

can you use pcie 40 gpu in 30 slot You can use a PCI-e card in a PCI-e slot but it will just work at PCI-e speed Not sure if you will really notice the speed