I'm planning to upgrade my drivetrain from Claris to 105. Can I still use my Claris crank with an 11-speed rear derailleur and cassette? Thanks.
No, that won't work! 8-speed and 11-speed systems use different chains. The 11-speed chain is narrower and doesn't fit the teeth of an 8-speed chainring.
Unless you get the 105 parts for very cheap, the usual advice here is not to perform this kind of upgrade. You'd probably find a whole new bike for the price of the items you'll need for a switch from Claris to 105.
Upgrading drivetrains several steps up the Shimano hierarchy is usually not cost effective.
Usually, the entire drivetrain groupset - including the crankset and likely the bottom bracket - and the rear wheel need to be swapped out. The wheel needs to be swapped because the freehub body for 11 speed systems is wider than that used for 8 through 10 speeds. Factoring in the cost of items such as a chain, cables and housing, bar tape and all the special tools you'll need cost adds up quickly.
Bare groupsets are expensive. They are effectively much cheaper when bundled with a whole bike. Often it's better to sell your current bike and purchase a new bike at the level you want.
If you are willing to spend time finding used used, good condition parts of the 10 speed Ultegra 6700 or 105 5700 ranges you can pull the cost down dramatically for almost the same benefit.