A lot of the war materiel donated by western powers is getting into Ukraine via Poland. Large trucks carrying vehicles loaded with weapons are driven to the border with Ukraine. The vehicles are then handed over to Ukrainian drivers. These take the weapons to Kiev and other cities.
Why don’t the Russians bomb these convoys? These convoys operate in the west of the country. Russia has no grip on the western part of Ukraine and it has not achieved air superiority. From time to time it uses missiles to attack depots in the west, but the damage inflicted is limited and the flow if weapons is not being stopped. The deliveries are becoming more streamlined. The US is helping Ukraine with air reconnaissance. This means that Russian planes are not safe over Ukrainian territory.
The US and its allies are probably also clandestinely flying weapons into Ukraine. Russia has no way of stopping this. It has very little idea of what is going on behind enemy lines. This must be extremely frustrating for the Russian army, since nothing can be done about it.