What type of encoding do .pem files use to store the key?

What encoding is the modulus in RSA cryptography stored as in a .pem file? Is it something similar to base64 of the modulus?

I can retrieve the modulus using the following:

openssl rsa -in pubkey.pem -pubin -modulus

But what does it actually do? If I have the product of two prime numbers(n) in decimal form then how should I encode it so that the rsa utility can decode it?