How do I make such a block?

I have a block, and if it touches the block, it doesn't fall, and if it doesn't, it will fall

i assume (because i didn't really understand) those are 2 different blocks and the falling one is custom. in this case, since it's conditional, you have to use the neighbour changes trigger, then detect the block you want and if it isn't there, then you can do "Spawn provided blockstate as a falling block at x, y, z" and "Remove block at x, y, z"