They just hitch a ride with a bird. Many trees, especially fruit trees, are planted by birds. The seeds of these plants are inside their fruit. Birds pick the fruit, fly away to eat, and drop the seeds. Quite often a fruit tree grows where a seed has been dropped by a bird.
Squirrels are very important tree planters.
Squirrels hide food away for the winter, especially acorns, walnuts, and hickory nut. Squirrels bury so many nuts that they don't always remember where they buried them. Inside every nut is a tree seed which, thanks to a forgetful squirrel, can become a full size tree. Other animals plant seeds, too. Plant seeds can cling to the fur of a dog, a fox, or a raccoon, only to fall off and grow in some distant location.