I remember americans left the equipment in afghanistan claiming that it was better to abandon then transporting it back. So they disabled them and took important items off of the vehicles and other items.
These equipment that the russians have left in hurry does not necessarily mean that all of tanks are in working order maybe they have disabled the internal electronic components. In no way I will believe they are all in working condition. The way Ukrainians are capturing russian hardware and tweeting that "captured tank in good condition" no one knows what good means here so fast. It may take hours to understand if this is in complete working order.
I don't know how much you know about Tanks......
Tanks are in working condition even if the optics are damaged or even destroyed. In fact, Tank have very high tolerant of damage, I can take 2 direct hit from the front and the tank can still drive and fire.
That's because tanks gun while complex, but it is a very basic design, you have a barrel, you have a beech, you have your primer, you have your round, and finally you have your trigger. It's basically works like a firearm, as long as you can load the round in, and ignite the primer, and the barrel is not obstructed, then your main gun WILL fire.
Engine is a more complex issue, it involved an engine and a gearbox, that too cannot be easily destroyed, and even if it did, you can still salvage or repair it
So basically, when you see most of the tank they capture have no external damage at all, it usually mean they are in functioning order, anything that make the tank cannot function would most likely showing the damage on the outside