I want to look at the source code for a function to see how it works. I know I can print a function by typing its name at the prompt:

> t
function (x) 
UseMethod("t")
<bytecode: 0x2332948>
<environment: namespace:base>

In this case, what does UseMethod("t") mean? How do I find the source code that's actually being used by, for example: t(1:10)?

Is there a difference between when I see UseMethod and when I see standardGeneric and showMethods, as with with?

> with
standardGeneric for "with" defined from package "base"

function (data, expr, ...) 
standardGeneric("with")
<bytecode: 0x102fb3fc0>
<environment: 0x102fab988>
Methods may be defined for arguments: data
Use  showMethods("with")  for currently available ones.

In other cases, I can see that R functions are being called, but I can't find the source code for those functions.

> ts.union
function (..., dframe = FALSE) 
.cbind.ts(list(...), .makeNamesTs(...), dframe = dframe, union = TRUE)
<bytecode: 0x36fbf88>
<environment: namespace:stats>
> .cbindts
Error: object '.cbindts' not found
> .makeNamesTs
Error: object '.makeNamesTs' not found

How do I find functions like .cbindts and .makeNamesTs?

In still other cases, there's a bit of R code, but most of work seems to be done somewhere else.

> matrix
function (data = NA, nrow = 1, ncol = 1, byrow = FALSE, dimnames = NULL) 
{
    if (is.object(data) || !is.atomic(data)) 
        data <- as.vector(data)
    .Internal(matrix(data, nrow, ncol, byrow, dimnames, missing(nrow), 
        missing(ncol)))
}
<bytecode: 0x134bd10>
<environment: namespace:base>
> .Internal
function (call)  .Primitive(".Internal")
> .Primitive
function (name)  .Primitive(".Primitive")

How do I find out what the .Primitive function does? Similarly, some functions call .C, .Call, .Fortran, .External, or .Internal. How can I find the source code for those?