Multiplayer Access is XBox Live Gold. This is what you need to play online with others.
Costs about 42 Euro a year.
You will need the XBox Live Gold for playing online (COD, Forza Horizon, MSFS etc.)
XBox Network (formerly XBox Live until March 21 and will be renamed with the next XBox update, Xbox Insider already see this name change) is the online access without the multiplayer, this comes -for free- with your xbox account and helps you download patches, make purchases online, store your game settings in the cloud and such.
The Xbox Game library (including the MSFS) is called XBox Gamepass, costs about 8,- Euro a month.
If you cancel that, you need to buy the game for yourself, online or on disc.
Both services, xbox live gold and gamepass together (also includign EA play, which standalone costs 4,- a month) is called the Gamepass Ultimate (13,- Euro/month). If you cancel that, you need to buy XBox Live gold to continue playing online games.
Gamepass Ultimate also includes gamepass for PC (the standard gamepass differentiates between game liberary for PC and for XBox), meaning, you can play a game both on xbox and PC/cloud gaming.
You can cancel any subscription easily and they run out after the last period you paid for. You can buy ultimate now for a month and cancel it immediately, meaning, after one month of playing the subscription runs out.