Dwarf Fortress\raw\objects\
in files that begin with "entity_"
In the case of the playable races, permissions are the lions' share of text within those files.
You can search in files for "PERMITTED_", of course; there will be hundreds of hits for each of the playable races.
Searching for: PERMITTED_
F:\Games\DF\MasterworkDF V6.2\Dwarf Fortress\raw\objects\entity_evil_warlock.txt: 1780
F:\Games\DF\MasterworkDF V6.2\Dwarf Fortress\raw\objects\entity_good_dwarf.txt: 4204
F:\Games\DF\MasterworkDF V6.2\Dwarf Fortress\raw\objects\entity_good_gnome.txt: 1488
F:\Games\DF\MasterworkDF V6.2\Dwarf Fortress\raw\objects\entity_good_human.txt: 3642
F:\Games\DF\MasterworkDF V6.2\Dwarf Fortress\raw\objects\entity_hermit.txt: 914
F:\Games\DF\MasterworkDF V6.2\Dwarf Fortress\raw\objects\entity_savage_kobold.txt: 1971
F:\Games\DF\MasterworkDF V6.2\Dwarf Fortress\raw\objects\entity_slaver_orc.txt: 1401
F:\Games\DF\MasterworkDF V6.2\Dwarf Fortress\raw\objects\entity_slaver_succubus.txt: 934
Found 18246 occurrence(s) in 25 file(s)
For example, you might find this block of text in the orc entity
[PERMITTED_BUILDING:ORC_FLETCHER]
[PERMITTED_REACTION:BOW_COMPOSITE_WOOD_ORC]
[PERMITTED_REACTION:BOW_COMPOSITE_BONE_ORC]
[PERMITTED_REACTION:BOW_COMPOSITE_IRON_ORC]
[PERMITTED_REACTION:ARROW_RUSTY_ORC]
[PERMITTED_REACTION:ARROW_COPPER_ORC]
[PERMITTED_REACTION:ARROW_OBSIDIAN_ORC]
[PERMITTED_REACTION:ARROW_IRON_ORC]
[PERMITTED_REACTION:ARROW_BRONZE_ORC]
[PERMITTED_REACTION:ARROW_STEEL_ORC]
[PERMITTED_REACTION:ARROW_GEM_ORC]
[PERMITTED_REACTION:ARROW_ENCH_BLOOD_ORC]
[PERMITTED_REACTION:ARROW_ENCH_CRYSTAL_ORC]
[PERMITTED_REACTION:ARROW_ENCH_MITHRIL_ORC]
Copy/paste that into another races' entity, and they'll have access to the orcish fletcher, and all its reactions. Now, whether or not your new custom race has access to all the needed materials to create the building, or all the reagents for every reaction is another story!
I don't really want to get too much into modding 101, you can find beginner resources on the wiki and people answering intermediate type questions up in the modding forum. good luck!