I'm assuming a "patch" is a group of commits. So would a "patch series" be similar to GitLabs notion of dependent MRs?
* Pile of commits - each individual commit doesn't matter as much as they all work combined. As a general rule, the only requirement for a valid patch is that the final version does what you say it does. Either the final result is squashed together entirely and then merged onto "master" (or whatever branch you've set up to be the "stable" one) or it's all piled together. Keeping the commit history one linear sequence of events is the single most important element here - if you submit a patch, you will not be updating the git hashes because it could force people to reclone your version of the code and that makes it complicated. This is pretty easy to mentally wrap your head around for a small project, but for larger projects quickly makes a lot of the organizatory tools git gives you filled with junk commits that you have to filter through. Most git forges encourage this PR system because it's again, newbie friendly.
* Patch series. Here, a patch isn't so much a series of commits you keep adding onto, but is instead a much smaller set of commits that you curate into its "most perfect form" - each individual commit has its own purpose and they don't/shouldn't bleed into each other. It's totally okay to change the contents of a patch series, because until it's merged, the history of the patch series is irrelevant as far as git is concerned. This is basically how the LKML (and other mailing list based) software development works, but it can be difficult to wrap your head around (+years of advice that "changing history" is the biggest sin you can do with git, so don't you dare!). It tends to work the best with larger projects, while being completely overkill for a smaller tool. Most forges usually offer poor support for patch series based development, unless the forge is completely aimed at doing it that way.
The second one makes sense, but I can't imagine actually working that way on any of the projects I've been in. The amount of work it would take just doesn't make sense. Can totally understand why it would be useful on something like the Linux Kernel though.
Under the original paradigm, the email list itself — and a (pretty much expected/required) public archive of such, e.g. https://lore.kernel.org for LKML — serves the same history-preserving function for the patch series themselves (and all the other emails that go back and forth discussing them!) that the upstream git repo does for the final patches-turned-commits. The commits that make it into the repo reference URLs of threads on the public mailing-list archive, and vice-versa.
Fun fact: in the modern era where ~nobody uses CLI email clients any more, a tool called b4 (https://b4.docs.kernel.org/) is used to facilitate the parts of the git workflow that interact with the mailing list. The subcommand that pulls patches out of the list (`b4 mbox`) actually relies on the public web archive of the mailing list, rather than relying on you to have an email account with a subscription to the mailing list yourself (let alone a locally-synced mail database for such an account.)
I'm thinking in terms of what I often see from people I work with, where a PR is normally made up of lots of small commits.
I think the point I always get stuck on is how small is "small" when we're talking about commits/patches. Like if you're adding a new feature (to anything, not necessarily the Linux Kernel), should the entire feature be a single commit or several smaller commits? I go back and forth on this all the time, and if you research you're gonna see a ton of different opinions. I've seen some people argue a commit should basically only be a couple lines of code changed, and others argue it should be the entire feature.
You commonly hear Linus talk about commits/patches having very detailed descriptions attached to them. I have trouble believing people would have time for that if each commit was only a few lines, and larger features were spread out over hundreds of commits.
Often, a change to a new working state is necessarily bigger than a couple of lines, or one of the lines has to get removed later.
I don't want to have to say, "Hmm, I wonder if this will work at the end of the file?" and spend a long time figuring out that it won't, then see that the problem is fixed later in the patch series.
Other people may have other preferences.
As per [0] merge commits are dropped:
Note that format-patch will omit merge commits from the output, even if they are part of the requested range. A simple "patch" does not include enough information for the receiving end to reproduce the same merge commit.
I originally thought it would use --first-parent (so just diff vs the first parent, which is what I would want) but apparently no! It is possible to get this behaviour using git log as detailed in this great write-up [1].
[0] https://git-scm.com/docs/git-format-patch#_caveats
[1] https://stackoverflow.com/questions/2285699/git-how-to-creat...