wat s differents bw top down assembly and bottom upassambly
If you were designing from a bottom up approach you have to have basic design of the component you want to model IE (ENGINE) (PUMP) ECT you can then model all the components and assemble them in the assembly.
If you were designing from the top down approach you would create, design and sketch in the top level assembly allowing your to create components and parts in the ASSY which do not have any geometry so that you can copy design sketched geometry to the components, parts and from there drive all of the components geometry with that sketch.
A top-down design project starts with the broad project specification in mind and putting that information in a centralized location. The design process progresses by communicating the centralized information to the individual parts.
In bottom-up approach, the individual parts are designed to fulfil the intended function and then put together to form assemblies of a product. After the assembly is in place, problems are identified and modifications to individual components are made. As components are independent of each other, if one component is modified, the related components have to be changed manually.
You can have a look at the following blog post for detail description.