Hi i was wondering how Saitama lost his arm in the virus!au ?
In Virus!Verse, Saitama suffered major wounds during a confrontation with a monster while he was still training. He fell into a deep apathy due to the pain of rehabilitation, and once he was finally used to living without his left arm, he was too deep into depression to care about continuing his training. He never became a hero, but thanks to Mumen and his cat Michi he eventually got to be in a healthier mental place.
Because he never became a guy who was a hero for fun, Genos never met him, and never had a drive to get involved with the H.A. until he discovered it was Metal Knight who was behind the Mad Cyborg incident. This eventually leads to him abandoning the name Genos and becoming Virus instead.