Hi @crescentrose17
Here's the [https://download.lenovo.com/consumer/mobiles_pub/lenovo_legion_5_pro_hmm.pdf|hardware maintenance manual] for the laptop. This manual covers the Legion 5 pro 16IAH7H Machine Types (MT) 82RF & 82SO laptops. The MT number should also be on the product information label on the bottom of the laptop if you wish to check which type yours is.
Go to p.42 to view the procedure to remove/replace the systemboard
Here's the [https://pcsupport.lenovo.com/us/en/products/laptops-and-netbooks/legion-series/legion-5-pro-16iah7h/82rf/parts/display/compatible?linkTrack=Caps%3ABody_SearchProduct&searchType=4&keyWordSearch=Legion%205%20Pro%2016IAH7H%20Laptop%20%28Lenovo%29%20-%20Type%2082RF|MT 82RF part list]
Click on M''achine Type → Select Commodity → Systemboards (14)'' to find the ***''part numbers''*** for the 14 systemboards that can be used in the laptop.
Once you have accessed the systemboard in your laptop, look for the part number which should be on a sticker somewhere on the board itself.
Once you have found it verify that it is on the list and if so search online using the ''part number only'' to hopefully find suppliers.
If you cannot find a supplier for your particular motherboard here's another option to consider. Here's a link to the [https://www.laptopserviz.bg/index.php/enview/lenovo-legion-5pro-16iah7h-lcfc-nm-e231-y550-rev2-0-boardview.html|schematics and boardview files] for the systemboard that can be purchased if you wish to try and find out what's wrong. Might be a cheaper option that buying a replacement board.