Question about TP-Link Deco BE65 (3-pack)

Well, as long as you are in the range of the WiFi. Being between 2 buildings, likely you are only going to use the 2.4Ghz band, because the 5 and 6 Ghz do suffer of obstacles. Unfortunately, the 2.5Ghz is also the band on which you will have the lowest speed 688 Mbps vs. 2882 Mbps of the 5 Ghz and 5765 Mbps of 6 GHz.
Another thing to check is not not have double or triple NAT, as you likely are going to have one router from the internet provider-Deco BE65 (1)-Deco BE65(2)-router-final device. It will work as long as you are in the range, but this is not an ideal setup.

Yes, it works. A router can be connected to a Deco BE65 via an RJ45 port to set up a wired network. The Deco uses the Wi-Fi mesh connection to the main device. Access point mode should be activated so that there are no IP conflicts. Stability depends on the signal quality - a clear line of sight improves the connection.

Yes, this is possible. You can connect a router via LAN to an RJ45 port on a Deco BE65 module to create a wired subnet. The Deco must be connected to the main Deco via Wi-Fi backhaul. Make sure that the Deco is running in access point mode (Deco app → More → Operating mode → Access point) so that there are no duplicate DHCP servers. Note that the speed and stability depends on the signal strength between the buildings.

Yes, you can connect a wired router to one of the RJ45 terminals on a Deco module to create a wired subnetwork. This means you can use the same internet subscription for several buildings without having to run an RJ45 cable between them. The RJ45 ports on each Deco Module can be configured to operate in WAN or LAN mode, allowing you to create a wide area network using the mesh system as a Wi-Fi bridge. Make sure the router you connect is compatible with 2.5 Gbps ports to maximise performance.