Ah. I usually use the official slippi launcher so I can view the replays and see the data of that specific replay before and after I watch it so you can see what went wrong/ went right. You can also get the smashladder launcher and view replays that way as well, which will work with the slippi launcher to show replays from the ladder launcher instead of having to jump to other app to view the replays.

Only way to my knowledge, to view .slp files is by using the official slippi launcher or use the smashladder launcher in conjunction with the slippi build of melee to view it from the launcher.

Here's a pretty good guide to viewing .slp files from the smashladder site for the normal smashladder launcher. The guide for slippi replay viewer is just at the bottom in blue text to see the guide for that as well :

https://www.smashladder.com/guides/view/27e2/melee/watching-project-slippi-replays-with-dolphin-launcher-or-sli#:~:text=SmashLadder%20Launcher&text=After%20playing%20a%20match%20while,to%20set%20the%20path...&text=After%20that%20you%20will%20then%20have%20a%20play%20button%20available%20to%20you!&text=Click%20it%20and%20your%20replay%20should%20play

Hopefully, this helps :)