For some games, each session is an adventure and the characters are all back at 'home base' between adventures. Adventurer's League and West Marches style games are often like this, but lots of other formats work too. In that case you're picking up the crew with each adventure, maybe Stabby the rogue is in jail and can't go on this West Marches expedition, or Electric Elliot has to fly off to solve a problem with someone else, or Cop McCoperson is being investigated by Internal Affairs and can't make the stake-out. With a lot of WM/AL setups, you're getting a new group every session anyway, so it's not even 'explanation'.
You can also just have the character go inactive, or mostly active and continue the adventure. If that character has a skill the party needs (like they have comprehend languages and you want that as an excuse to lore dump, or they are the one who can cast teleport circle to get everyone to the next spot) the DM can just have them do it, and otherwise the character is busy or sick or offscreen.
It's possible to have another player control the character, but it's pretty clunky, requires some agreement about how that works (can the character die? Can the current player use up limited-use consumables or accept bad things for the PC?), and generally isn't needed.
If someone is really so lacking in suspension-of-disbelief that this ruins the session you could just skip a session, but most people would rather shrug off a minor 'realism' point in favor of actually gaming.