It's normal for hair to go through lots of periods of increased or decreased shedding over the years. I imagine for people who are on hairloss meds they may not have periods of increased loss, or it may be subtle enough to not notice.
With washing your hair, the reason you see more hairs falling out if you haven't washed your hair in a few days is simply because those hairs have been sitting there ready to fall, but haven't had that little push needed that a hair washing did. They were going to fall out anyway. It's like if you have trees growing in your yard and you rake the leaves every day you are going to have less leaves to rake than if you waited a few days to rake them.
Don't get too caught up with 100 hairs a day. It's just a general guide. If someone with a full head of hair loses 100 hairs per day he may not even notice, but if a NW 6 is losing 100 hairs per day then that will be very noticeable because 100 hairs is a much greater percentage of the remaining hair on his head.
As far as Covid causing hair loss a few months later, yes that's a possibility, but it's not just Covid. Any severe sickness, disease, or something that stresses your body and immune system can cause temporary hair loss. That loss shows up a few weeks to a few months later and then grows back. This happens to women who give birth for the same reason. Some of them experience temporary hair loss a few months afterwards.