At the DCP page you read about sending your DCP fysically on a disk. Ever more festivals and cinemas accept internet based transfers. Some festivals even deploy their own dedicated upload service. If a festival accepts internet based delivery but does not offer an upload service, you may consider using one of the available transfer options. Don't expect this to be a cheap alternative for fysical delivery, some may be faster though.
There are countless services providing file transfers. I did not find a reliable free service capable of handling file sizes above 5 GB. Typical DCP sizes are much larger, so you will need a paid subscription if you choose to transfer your DCP. If you have a paid file sharing account like WeTransfer, you may try to use that at your own risk. You better first test if files arrive unharmed. Do not use Dropbox, OneDrive or Google Drive file sharing for large file sizes, i had numerous failed downloads with those services. When choosing from available transfer services best results i get and what people report are with Filemail and Aspera. Filemail is reported as slow but reliable. Aspera is a much faster service (by IBM) and needs dedicated software at both the sender and the receivers sides. The only "free" option you might consider is setting up a ftp server yourself. If you own a NAS you may find a useful ftp file sharing service on that device. First of all check at the festival or cinema what they accept.
Filemail has a pro option at a fee of 120 euro a year (oct 2022). The free option only works for file sizes up to 5GB. The 7 day free trial option might help you if you need a transfer only once. Connecting to a faster Filemail server can speed up things. It has apps for Windows and macOS (and mobile).
Aspera comes at a price of over 700 dollar a year. It also has a 1 US dollar per GB option for a maximum of 10 users. Aspera has a 30 day trial option. Be aware that it will be necessary to send clear instructions to receivers about how to get your DCP using Aspera. It has apps for Windows, macOS and Linux.