Daniel Wroclawski over at Consumer Reports has an excellent Matter FAQ from an end consumer point of view.
With regard to the Arlo announcement, it’s important to note that matter has not yet included any cameras: this is just Arlo saying they want to be part of deciding what any future camera standard might be.
From the CR report:
Which Types of Smart Home Devices Will Work With Matter?
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At launch, Matter will support lighting products (plugs, bulbs, switches, etc.), door locks, thermostats and HVAC controllers, blinds and shades, home security sensors (such as door and window sensors, motion sensors, and smoke and carbon monoxide detectors), garage door controllers, wireless access points and bridges, and televisions and streaming video players.
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Future iterations of Matter are expected to add support for home appliances, robot vacuums, and energy management products (such as solar panels, EV chargers, home battery packs). According to Chris LaPré, the CSA’s head of technology, cameras are likely to be included in a future version, thanks to camera manufacturers like Arlo, Eve, TP-Link, and Wyze joining the CSA, but no formal effort has started yet.”