Hello Doraeemon1, I am not familiar with the Steam install. However I found this straightforward utube (https://www.youtube.com/watch?v=7je6TrAvJfc) and notice the two steps – quick install of the main program within Steam and then when the program launches and wishes to download the REALLY BIG CONTENT – you can select another drive and another subdirectory. CAUTION: 132Gb drive may be insufficient! After purchasing a second 1Tb SSD that is currently dedicated soley to MSFS 2020… I am nearly at 300GB! Yes, I’ve installed all regions, etc. There is a LOT of information in this forum and online and in youtube about installing and migrating and managing MSFS across multiple drive. I wish you well.
Mark