When you load your existing save, you have one of two options to choose from.
When you choose to play a solo session, it will load your file from the last time your game saved. Even though you are playing in a solo session, there is a chance a random player may join your session or you may stumble across other players as your travel through the galaxy.
You can still play the game on your own and have that single-player experience. By default, if you do, then you may come across other players. Other people might join you, you might bump into random strangers
When you choose to play a random multiplayer session, it will temporarily load your file in the star system the other player is currently in. When you exit out of a multiple session and return to a solo session, your game will load in your last saved location on the solo session.
You will only be able to play with other players who are on the same difficulty as you (standard, survival, creative, or permadeath).
These modes really change the type of game, so, [in multiplayer] you’ll all have to play in the same mode – it would be pretty annoying to be trying desperately to stay alive in ‘survival’ mode and somebody else is just breezing around in ‘creative’ mode.