So I just kept Ctlr+clicked(vscode) my left mouse on "+" until add_int(x, y)

## integer comparisons ##

(<)(x::T, y::T) where {T<:BitSigned}  = slt_int(x, y)

(-)(x::BitInteger)                    = neg_int(x)
(-)(x::T, y::T) where {T<:BitInteger} = sub_int(x, y)
(+)(x::T, y::T) where {T<:BitInteger} = add_int(x, y)
(*)(x::T, y::T) where {T<:BitInteger} = mul_int(x, y)

It seems to me that add_int is a function however I don’t know where to find how it is implemented

it’s an intrinsic defined in C. You can see the list of them here, and the implementations here and here.