How to Gen RSA 2048 to pfx, der, cer file ....

openssl genrsa -out private.key 1024
openssl req -new -x509 -key private.key -out publickey.cer -days 365
openssl pkcs12 -export -out public_privatekey.pfx -inkey private.key -in publickey.cer

Step 1 – generates a private key

Step 2 – creates a X509 certificate (.cer file) containing your public key which you upload when registering your private application (or upgrading to a partner application).

Step 3 – Export your x509 certificate and private key to a pfx file. If your chosen wrapper library uses the .pem file to sign requests then this step is not required.

Other Tips:

Create a private-public key pair.

openssl req -x509 -newkey rsa:2048 -keyout private.key -out public.cert -days 365

Optionally, combine the pair into a single file.

openssl pkcs12 -export -inkey private.key -in public.cert -out certificate.pfx

This results in the following files.

private.key
certificate.pfx
public.cert

https://www.sslshopper.com/article-most-common-openssl-commands.html