Where are the hyphenations patterns? Are they the same for xelatex and polyglossia as for latex and babel? (Links to information on how to modify them would also be appreciated.)
You can find them, in TeX Live 2015, in
The distinction should not be made between
polyglossia, but between 8-bit engines (like
pdftex) and Unicode engines (XeTeX and LuaTeX). However, up to a certain extent, all engines can share most of the hyphenation patterns through a clever translation process.
If you want to study how hyphenation in TeX works, you must read Appendix H of the TeXbook; something about it is also in TeX by Topic.
Modifying a set of hyphenation patterns is quite easy: they're text files, after all. Applying meaningful and coherent changes is a very different matter. Some of the sets are made from grammatical rules (it's so for Italian) and they are quite simple to read and modify. Changing the patterns for British English without a deep knowledge of
patgen is hopeless. The program
patgen is able to distill patterns from lists of already hyphenated words; it requires expertise that just a few people in the world possess (I'm certainly not among them).