How do I create a csr and certificate?

Question

I'm doing a project where I have to generate a digital certificate and then add that certificate to pdf files. The key pair generation process is done on a mobile (written in with flutter) and I (correct me if I'm wrong) thnk that I would then have to send a csr or a the public key to a server to generate a certificate.

The problems that I am currently stuck with is what the whole process should look like? Do I generate a CSR on the mobile side, then send that csr file to the server that I created which has a python script to generate a certificate?

However, from the code I got from online for generating certificates, it seems like it requires the user's private key. How do I approach this problem? Here is the code I got which I can't remember where I got it.

Note: I already am able to generate a digital certificate (signing a hash)

from OpenSSL import crypto
import os
import sys
import datetime
import whois

dir_path = os.path.dirname(os.path.realpath(__file__))
if getattr(sys, 'frozen', False):
    dir_path = os.path.dirname(sys.executable)
elif __file__:
    dir_path = os.path.dirname(__file__)
currentDir = dir_path.replace('\\', '/')

#Variables
TYPE_RSA = crypto.TYPE_RSA
TYPE_DSA = crypto.TYPE_DSA
HOME = currentDir
now = datetime.datetime.now()
d = now.date()

#Pull these out of scope
cn = input("Enter the Domain: ")
key = crypto.PKey()
keypath = HOME + "/" + cn + '-' + str(d) + '.key'
csrpath = HOME + "/" + cn + '-' + str(d) + '.csr'
crtpath = HOME + "/" + cn + '-' + str(d) + '.crt'

#Generate the key


def generatekey():

    if os.path.exists(keypath):
        print ("Certificate file exists, aborting.")
        print (keypath)
        sys.exit(1)
    #Else write the key to the keyfile
    else:
        print("Generating Key Please standby")
        key.generate_key(TYPE_RSA, 4096)
        f = open(keypath, "wb")
        f.write(crypto.dump_privatekey(crypto.FILETYPE_PEM, key))
        f.close()

    #return key

generatekey()

#Generate CSR

def generatecsr():

    print ("How would you like to generate csr data?\n" \
          "1) CQB (For Self-Signed Certs).\n" \
          "2) Specify your own.\n" \
          "3) Attempt Whois Look")

    option = int(input("Choose (1/2/3): "))
    if option == 1:
        c = 'US'
        st = 'California'
        l = 'Berkley'
        o = 'CQB'
        ou = 'Network Operations'
    elif option == 2:
        c = input('Enter your country(ex. US): ')
        st = input("Enter your state(ex. Nevada): ")
        l = input("Enter your location(City): ")
        o = input("Enter your organization: ")
        ou = input("Enter your organizational unit(ex. IT): ")
    else:
        print("Attempting WHOIS Lookup")
        w = whois.whois(cn)
        c = str(w.get('country'))
        st = str(w.get('state')).lower().title()
        l = str(w.get('city')).lower().title()
        o = str(w.get('org')).lower().title()
        ou = 'Network Operations'

    req = crypto.X509Req()
    req.get_subject().CN = cn
    req.get_subject().C = c
    req.get_subject().ST = st
    req.get_subject().L = l
    req.get_subject().O = o
    req.get_subject().OU = ou
    req.set_pubkey(key)
    req.sign(key, "sha256")

    if os.path.exists(csrpath):
        print ("Certificate File Exists, aborting.")
        print(csrpath)
    else:
        f = open(csrpath, "wb")
        f.write(crypto.dump_certificate_request(crypto.FILETYPE_PEM, req))
        f.close()
        print("Success")

#Generate the certificate
    reply = str(input('Is this a Self-Signed Cert (y/n): ')).lower().strip()

    if reply[0] == 'y':
        cert = crypto.X509()
        cert.get_subject().CN = cn
        cert.get_subject().C = c
        cert.get_subject().ST = st
        cert.get_subject().L = l
        cert.get_subject().O = o
        cert.get_subject().OU = ou
        cert.set_serial_number(1000)
        cert.gmtime_adj_notBefore(0)
        cert.gmtime_adj_notAfter(315360000)
        cert.set_issuer(cert.get_subject())
        cert.set_pubkey(key)
        cert.sign(key, "sha256")

        if os.path.exists(crtpath):
            print ("Certificate File Exists, aborting.")
            print (crtpath)
        else:
            f = open(crtpath, "wb")
            f.write(crypto.dump_certificate(crypto.FILETYPE_PEM, cert))
            f.close()
            print ("CRT Stored Here :" + crtpath)

generatecsr()

print ("Key Stored Here :" + keypath)
print ("CSR Stored Here :" + csrpath)