Everything important is integrated inside CPU. pcie, ram, usb 3.0, sata, HD Audio, even RTC/SPI/I2C/SMBus and LPC are on die. You can make perfectly functional system with just an AMD CPU alone. Those CPUs are full SoCs now.
xx20/50/70 is all market segmentation upsell bullshit. AMD still tries to pull a fast one linking CPU/CPU features to the number on the chipset. Sometimes giving up after a backlash :
https://www.itworldcanada.com/article/amd-zen-3-processors-will-be-incompatible-with-400-series-and-older-motherboards/430513
https://www.tomshardware.com/news/amd-reverses-course-will-enable-zen-3-support-on-b450-and-x470-motherboards
other times just sticking a tip in like in the case of AMD Smart Access Memory totally requiring 500-series chipset despite being just a fancy marketing name for standard PCI Express Resizable BAR support. Already shipping disabled for 2 prior generations before being announced as 5000 exclusive. https://www.extremetech.com/computing/320548-amd-will-support-smart-access-memory-on-ryzen-3000-cpus-for-gaming
Some CPUs can indeed function without a "chipset", but AM4 needs one, at the very least for some "glue logic"
For example Threadripper's System-on-chip die has everything so that a chipset is not needed, but AM4 processors still need the chipset.
Either way, the processors do have a few peripherals, a couple sata ports, a few usb ports, pci-e lanes going to a m.2 connector, but the chipset enhances the platform.
The chipset connects to the cpu using a pci-e 3.0 x4 (or pci-e 4.0 x4) link and has sata and usb controllers and creates additional pci-e lanes which can be routed to pci-e slots or extra m.2 connectors or peripherals.
For B450 chipset, there's 6 pci-e lanes, for x570 there's 8 pci-e lanes.
x570 is more or less adapted from the io die of threadripper, and supports pci-e 4.0 and has a higher power consumption, therefore if you do use pci-e 4.0 peripherals, it needs to be cooled, so most motherboards have a fan.
B550 is different chip, only supports pci-e 3.0 (lanes from cpu remain pci-e 4.0 , lanes with go to 1-2 pci-e x16 slots for video card and m.2 connector) and doesn't need active cooling.
B520 is a cut down version of b550 with artificial restrictions and limitations, but it's targeted at budget/office motherboards, and nobody lies to you about what you're getting.. not everyone needs 10 usb ports, 6 sata , more than 2 slots of ram, overkill vrm .. a well optimized 3-4 phase vrm with decent power stages absolutely can power a 65w tdp cpu with integrated graphics for an office pc, especially if the chipset limits / disables overclocking. Not everyone needs a 12-14 phase vrm on their motherboard.
pci-e 4.0 support was artificially restricted on B450 and boards before B5xx chipsets because they couldn't guarantee it would work well with all previously released b450 motherboards.
pci-e 4.0 requires better signal quality, the trace lengths must be a certain maximum length, it's said that even the pcb material is improved on b5xx motherboards to support pci-e 4.0 properly. A lot of decent B550 motherboards moved to using 6 layers or more, to separate the pci-e routing and the memory traces from other layers.
as for that smart access feature not being supported on pre b5xx boards ... again, it has to do with how much validation, how much testing, how much you add to the agesa, how many bios updates you add ... how much pain in the a** you get for something that most users with those boards won't use.
Whoever has a video card that supports that feature AND a processor that supports it, has the money to upgrade a b450 motherboard.