How could someone manually create a transaction on a blockchain?

I've been thinking, it must be possible for someone to create a transaction manually without the use of an already established wallet software, correct? So it got me thinking, how exactly might one go about doing this? If I have a seed phrase and funds on any given blockchain contained in a wallet generated by that seed phrase, how can I create a transaction manually on that blockchain so that even without having access to an exchange or wallet software, I could move the funds? What would be the requirements? Would it basically be absolutely necessary to create at least a barebones wallet software from scratch, or is there some kind of simple command/script that could be run to create the transaction ???? hopefully this question isn't too rookie for you guys or anything...