these plug into a AC powerpoint and detect the main router's WiFi signal and re-broadcasts it at a higher signal strength and thus acts as a WiFi booster
MESH WiFi is a new system based on WiFi 6 (802.11ax) which adds OFDMA, uplink MU_MIMO, Spatial frequency reuse, dynamic fragmentation technologies
each node or satellite extender communicates with one another to amplify your WiFi signal and to extend its WiFi coverage. This allows you to add a number of WiFi satellite extenders into the system whilst retaining the MESH WiFi SSID and password for easier access to the system as one moves from one part to another
they can allow for faster speeds, smoother reliability and greater wireless coverage
generally require a MESH compatible main WiFi 6 router and each node must have WiFi access to another node
it replaces the WiFi 5 main router's network system with its own MESH system
easier to setup than multiple extenders
better performance and better redundancy
more expensive but you no longer need your main router as one of these will replace it
probably only needed for larger homes
if you need a system to get through brick walls or have a weathersealed outdoor option, or you need a ethernet direct connection to a node for gaming, the Netgear Orbi is better than Google Nest or Linksys Velop for these needs
if those needs are not important, the Google Nest is better for a small home or unit, and the Linksys Velop is better for a larger house as it is far more consistent than the Google Nest although the app is problematic
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for those who are a bit tech savvy, consider an industrial
PoE Mesh access points although these generally require connection to a PoE ethernet cable at each access point (or IP camera or IP phone) but this makes it much easier to install access points in ceilings where the WiFi will be more optimal and IP cameras on external walls.