Is all of encryption based on xor? For a long time I've been under the impression that all of encryption uses xor at its basic level.

Is this true? What other method could be used to encrypt plaintext with a key? Asymmetrically, as while as symmetrically?

There are hundreds of ciphers, aes, blowfish, ect. They are all different, but do they all rely on xor?

It is a dumb question, however it was never asked (at least on google).
You don't have to use XOR, but rather, it tends to be convenient.  One of its convenient properties is that it is its own inverse.  Also, XOR implements addition in $\mathbb F_{2^n}$, making XOR a key component of working in finite fields of characteristic $2$, if you want to use them in an algorithm.

The "one-time pad" is usually defined as taking a random key the same size as the input and XORing them together to produce the ciphertext.  If you changed XOR to addition, whether $\mod 256$ or considering the entire input to be one large number, it would still be perfectly secure.  It just wouldn't be its own inverse - decryption would require subtraction instead of addition.

RC4 was considered secure for years, and though somewhat broken in modern times, it can serve as an example.  RC4's algorithm revolves around swapping bytes in an S-box, then determining a byte to XOR with the plaintext.

If you modified RC4 to do $\mod 256$ addition instead of XOR, it would have the same security as standard RC4.  However, it then wouldn't be its own inverse; decryption would require an RC4 that uses $\mod 256$ subtraction.