I relate with what everyone is describing.
I think if you have to ask what a covid crash is, then you haven't experienced one yet. For me they are sudden and intense. When I overdo physical or mental activities I will get pre symptoms of visibly shaky, cold, headaches like my brain is expanding. At the worst I will feel a sudden vibrating feeling directly behind my sternum and almost pass out. I will not be able to think clearly. When I push myself this far, I will be unable to pick my head up the next day and have body swelling. Usually after a day of rest i will recover. Staying hydrated (3 litres of water a day) and taking it easy help. If I don't get enough sleep it makes everything worse.