To get a custom item, you will have to use a creative saved hotbar.
You create an item in your own creative world, save it to a hotbar, and then use said hotbar to get it in creative!
If you would like to use any of the item models, they will be listed on the pages of an item. For instance; if you wanted to the Sawed-Off Shotgun model, it is listed on the page! Use the value in the "item_model" component when giving yourself the item in your own creative world.
Item tags are a bunch of custom data values that can be added to items to change there behavior!
Double Jump
double_jump: 1b
Any item you wear with this tag will allow you to have a secondary mid-air jump!