Question about EOS block producers

So I undertand how on steem witnesses are selected and how we are using more than 21.

I checked a block explorer and was able to find all the producers and noticed that only about 60 or so are earning even though a few hundred seem to exist.

can someone explain to me how block producers are chosen and how other than the 21 main producers earn or become part of the round of block producers.

also how is this different from steem and bitshares and why were these differences chosen.