Hi. This is a topic that comes up a lot on this forum. You can do a search for the answer for it and you'll have a very long list to read from.
Specific BIOS files, with specific names and specific ROMs for each...etc
Here is my answer to a recent post with the same question. Please read not only the procedure above, but this one as well. That's the BEST answer anyone can give you.
"once you have the correct BIOS files in place and the emulator(s) working. I highly suggest taking some time to go through this build setup for lr-atari800/5200.
Without it, your systems will work, but look nothing like they should, especially the a800 side."
https://retropie.org.uk/forum/topic/22392/lr-atari800-5200-artifacting-basic-and-other-guidance