The most basic scarf for joining two timbers together is the basic half-lap joint.
One timber has half its depth cutaway on the top, and one timber has half its depth cutaway on the bottom, and these two pieces lap over each other.
This is an exploded view of a simple half-lap of two 8×8 sill timbers.
When timbers are directly supported with a foundation, then this type of joint can be used.
It is not the strongest, just the simplest to cut.