Air carrier First Officer reported the Captain flew an unstable approach that was too low to the ground. It was not until the 4 red PAPI lights were in sight that the Captain corrected the flight path.

Narrative

I was pilot monitoring. We were setup and briefed for the RNP-Z straight-in for Runway XX. The airport was using Runway XY and we were told to expect a visual approach to Runway XY. The weather was clear with calm winds and excellent visibility. The Captain who was pilot flying turned the flight director and Autopilot off and elected to hand-fly a visual approach from a right downwind for Runway XY. I confirmed that the landing data was the same for both Runways XX and XY.Abeam the airport in the downwind the captain began a slow descent from the 1500 ft. AGL pattern altitude. While on base; I was looking for traffic on final as well as traffic in the left downwind when I heard the '500' callout. I looked at the Primary Flight Display (PFD) and saw that we were at 500 ft. AGL and still in a turn on the base leg. At this point I believed that we flying an unstable approach. We were on a right base for Runway XY and there was traffic on a left downwind for Runway XY at the same time somewhere ahead of us that I did not have in sight. I was concerned that if I called for a 'missed approach' while on a base heading we might climb ahead into the path of the other aircraft in the downwind and cause a mid-air collision. I decided that the safest decision was to not call a missed approach and allow the Captain to continue the turn to final. I did call out 'you're too low' two times and while I did not hear a verbal response; I did notice the Captain raising the nose and slowing the descent rate. While still in a bank to final; I noticed the radio altimeter (RA) was ranging between 230 and 270 ft. We came close to a radio Tower (a charted obstruction); and at one point we were so low that I was having difficulty seeing the runway environment. The airplane went wings level around 200 ft. AGL with 4 red lights on the PAPI which the Captain corrected for once on final. The rest of the landing was uneventful.While on the ground a ramp employee in ZZZ commented to me about the approach and said it looked like we got close to a radio Tower (the charted obstruction to the southeast of the airport). I also had communication with another airline deadheading Captain who stated that her and her First Officer both had concerns with how low we were and the stability of the approach. She stated they were looking at the replay of our flight on a flight tracking application to see how low we were.The Captain and I had very little interaction after the flight. I believed that flying so close to the ground and to obstructions was not safe or standard practice; and felt it was possible it violated a FAR. In ZZZ I reached out to a trusted check airmen for advice on how to handle the situation. After that conversation; I decided a report was appropriate. Upon returning to ZZZ1 from ZZZ I further expressed my concerns to the Captain and told him I wanted to file a report for the approach into ZZZ because of how the pattern was flown and how low we were. I also told him that another airline crew members and a ramp agent expressed concern as well. Based on that interaction; I contacted a Duty Officer.I had a moment of tunnel vision looking for the traffic on final and in the downwind. Once I realized how low we were; it presented a hazard to go around while on a base heading due to other aircraft in the pattern. If I had realized earlier that we were going to be so low I could have been more assertive in having the plane extend downwind and maintain altitude for a longer final. There is also no training or guidance I'm aware of on how to go missed when you're not aligned with the runway and flying a visual pattern. It was unclear to me how best to handle the situation given the unusual circumstances. Once on final with 4 reds on the PAPI I should have called for a missed approach based on the entire pattern flown. I made call outs about our altitude being low; but looking back I wish I had been more assertive and used more urgency in my tone of voicedue to how low we were flying.