can i load the dota 2 map into hammer/workshop?

[timber hull]

If you open the Asset Browser, then search for "dota" while filtering maps on the type filter, you should see the regular dota map

I see alot of people useing the standart dota 2 map for there Arcade custom games, i dont beleave they build the game again from scratch right lol?
Of course not.

When you make a mod, if you don't do anything, you're playing on regular Dota. The modification begins from that point - you start changing the map, the heroes, the items, the abilities, the game rules.. etc.