Gaining research experience after undergrad
Cold emails are almost certainly going to be ineffective. The recipient doesn't know you or know anything about you other than what you say.
If you really want to do something, then get into a graduate program.
Failing that option, talk to one or more of your undergraduate professors about your needs. Perhaps they have something that you could work on with them.
But your previous professors hopefully also have contacts in academia elsewhere and they can recommend you to a colleague and it will be more likely to be acknowledged. If I get an email from you, I will probably ignore it. If I get an email from a previous contact/colleague who is recommending you to me then I will certainly follow up on it, even possibly by setting up a chain of recommenders.
As far as &quot;credit&quot; goes, I think that depends mostly on what impression you give in any project you attach to and what the PI of the project is willing to say about you. A line in the CV is fine, but a recommendation from an academic may carry much more weight.
You don't say what your long term goals are in the question. I suggest that you think about that. Where would you like to see yourself in five years? Ten?