what's the difference between these 2 select fields?

In general you can find the double arrow on select field with records sourced , and single arrow on lists. But again that depends on preference also. Second difference to note is you can use select from a list/search button in above while with single arrow you don't. Also in double arrow values are suggested and sourced based on your inputs dynamically. while in single arrow dropdown all values are present at any particular time.

this is how they look in the sandbox, but in production, the first one is like the second one and I can't set the value for the first one via client script.

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Be real careful here if you're playing with CS, I think the bottom dropdown might not be nullable, but the top one is nullable... aka if you want to put a "blank value" I think the top one is not restricted, but in the bottom if blank doesn't exist in the list it'll error/ or kinda.. more like.. it doesnt work

There double arrows appear depending on the users preferences for how long dropdowns can be. On the general tab in your preferences...

Maximum Entries in Dropdowns

, one of your environments probably has way more entries or the preferences are different.

yes, I forgot name of this preference. Thanks @Sandii

Thanks all