how are 开办 and 成立 different? they both have the meaning of “set up” or "establish " right?
In most situations, I would translate 开办 as “to open”; “to start”, and 成立 as “to set up”; “to establish”; “to found”.
You would 开办 a shop or school, but 成立 a nation or organization.
成立 can also have a separate meaning of “to be tenable”; “to hold water”, often used in the negative, e.g.; 这个论点不能成立.
Both have more 同义词.
开办：兴办，创办，创立
成立：建立，树立，创建
Most of those 同义词 are 同义词 with each other, and there are still more… 创设… 设立…
You can get an idea of what shades of meaning they have by looking at the literal meaning of the individual characters, and then by encountering them in use.
Check jukuu.com for example sentences with translations.