Can’t speak for newer rigs. My 2007 had a no name DVD/surround sound system that died after a few years. It was installed in a fairly small slot under the TV. I measured the height/width/depth of the opening and went to Best Buy. They had several 5.1 Blu-ray home theater systems, but only one that would fit in that slot. So I bought that. It came with new speakers (as they all did). The base unit fit perfectly. But the speakers were slightly larger and I had to enlarge the openings in the cabinet where those were installed. Not too big a deal. And sound is much better than the OEM unit. I did use the existing speaker wiring to connect the new speakers.
The new system has FM radio but not AM as the old one had. But we never used either so no problem there. And the old system had speaker outputs for two additional speakers in the bedroom. The new one does not have those outputs. But again we never used the bedroom speakers anyway.