The reason is that the linkup delay timer is a feature that delays bringing downstream VSX links up, following a VSX device reboot or an ISL flap. The linkup delay timer has two phases: initial synchronization phase and link-up delay phase.
The initial synchronization phase is the download phase where the rebooted node learns all the LACP+MAC+ARP+STP database entries from its VSX peer through ISLP. The initial synchronization timer, which is not configurable, is the required time to download the database information from the peer.
The link-up delay phase is the duration for installing the downloaded entries to the ASIC, establishing router adjacencies with core nodes and learning upstream routes. The link-up delay timer default value is 180 seconds. Depending on the network size, ARP/routing tables size, you might be required to set the timer to a higher value (maximum 600 seconds). When both VSX devices reboot, the link-up delay timer is not used. Therefore, by configuring the linkup delay timer to include LAGs 101 and 102, which are part of the same VSX device as LAG 201, you can ensure that both devices have enough time to synchronize their databases and form routing adjacencies before bringing down their downstream links.

The show arp command displays the ARP table for a specific VRF or all VRFs on the switch. The ARP table contains the IP address to MAC address mappings for hosts that are directly connected to the switch or reachable through a gateway. If the client device is connected to another switch by VSX LAG, the ARP entry for the client device will not be present on the primary switch unless it has communicated with it recently.
Therefore, to find the IP address of the client device, the administrator should run the show arp command on the secondary switch in the VSX pair, specifying the VRF name that contains the client device's subnet.

To onboard a new UXI in an existing environment with 802.1X authentication, you need to use the UXI app on your smartphone and connect the UXI via Bluetooth. The UXI app allows you to scan the QR code on the UXI sensor and configure its network settings, such as SSID, password, IP address, etc. The Bluetooth connection allows you to communicate with the UXI sensor without requiring any network access or cellular connection. The other options are incorrect because they either do not use the UXI app or do not use Bluetooth.

To onboard a new UXI in an existing environment with 802.1X authentication, you need to use the UXI app on your smartphone and connect the UXI via Bluetooth. The UXI app allows you to scan the QR code on the UXI sensor and configure its network settings, such as SSID, password, IP address, etc. The Bluetooth connection allows you to communicate with the UXI sensor without requiring any network access or cellular connection. The other options are incorrect because they either do not use the UXI app or do not use Bluetooth.