what is the difference between top down and bottom up approach?

Bottom-up

If you don’t know what you are doing, don’t plan yourself ahead of time or/and if you work alone:
•bottom-up choice > From small to big will be your mantra. Start designing a wheel and only after it is done, figure out where it’s going to be connected with an axle. You want to experiment things.

Example 01:

Create this support without knowing exactly where it will be fixed. After you finish you part desing, you will start to be concerned about creating the supoort that is missing.

Top-down

If you know what you are doing, if you plan yourself ahead of time and/or if you work with a group:
•Top down choice> You normally have a skeleton/layout with some guidelines. Based on that, you design.

Example 02:

Create this mechanism using a skeleton/layout. Just remember that you can use a lot of different things as a contextual reference (planes, surfaces, notes).

Top down:
You have an existing part or assembly. You then design another part to fit with it. These relationships are often used to keep multiple parts related to each other.
Change the location or size of threaded holes in a body, and the location of the matching holes in a cover will update to match. Even the bolts/screws can be configured to match the changes.

Bottom up.
You work on each piece independently. Relationships between the parts do not exist. Changes to one part do not cause a change in other nearby components.

Both methods have pros and cons in different situations.

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.

http://caeuniversity.com/2015/03/16/bottom-up-and-top-down-approach/