The whole floor needs to b replaced under everything in my bathroom on the second story of a 1930s home the bath tub feels like its falling when u take a shower and it leaks into the basement causing mold, & just recently the toilet has started leaking makeeing the floor around it really soft and constantly stinking like fresh amoponia

Call a professional... flooring and subflooring is one thing... but you might have rotted floor joists and that is beyond the average homeowners abilities.

I'm not sure what your question is, exactly, but if the toilet and tub are leaking, then you need to fix the plumbing, as well as doing the floor.

What is the flooring? Wood? Tile or vinyl over wood? Decide how you are going to replace it first. Go shopping to get ideas and prices. If you put in tile, you will probably want to put down concrete board first to get a solid base. If things are really bad, you might have to fix some joists first.

I would start by removing the toilet. You'll have to do that anyway to replace the floor, and that way you might be able to see what is leaking. You should have some access to the tub's plumbing. There might be a closet on the other side of the wall (assuming a built-in tub) with a panel that permits access.

When you begin to investigate, post back with some specific questions so that people can better help.