MacPro (2019) and 2.5" hard drives?

yes. There are two SATA plugs, a power plug and a powered USB-A shaped USB-3 connecter inside, near the top. This is one example of a kit you can buy to exploit that:

https://www.sonnettech.com/product/fusion-flex-j3i/overview.html

The reason Apple walked away from SATA connections for its internal drives is that SSD drives are now potentially faster than SATA-3 ability to move the data at highest speed. So that slows it down a little.

gordonTEDster wrote:

Is it possible to install 2.5" hard drives or SSDs in to a MacPro (2019), in addition to the system SSD from Apple?

Yes. This is the cheapest solution I've seen that will allow either 2.5" HDs or SSDs to be mounted in a 2019 Mac Pro. Everything needed is included. I have no experience with this product so the mileage may vary. https://www.amazon.com/dp/B097DSGGRX/?coliid=I3APVJ9RLJES0C&colid=3GM2RTC7IJXFL&psc=1&ref_=lv_ov_lig_dp_it