Dell fans don't always have standard wiring. Case fan is the cpu fan. There is also sometimes a HDD fan underneath the hard drive.

NY290 Hard Drive Fan Assembly 745 755 SFF

https://www.dell.com/support/home/en-us/product-support/product/optiplex-755/docs

https://downloads.dell.com/manuals/all-products/esuprt_desktop/esuprt_optiplex_desktop/optiplex-755_user%27s%20guide_en-us.pdf

https://www.amazon.com/dp/B08G8682S1

HDD fan header under HDDCPU Fan Header

Here the photo of the mb manal (pdf specs at page 7) 

Here a photo of the motherboard:

..and a zoom where i aspect to find the fan power plug (instead there are two eSata connectors):