I do this all the time and it's actually incredibly easy, I'll give you a quick guide which I hope is a little more in depth so it's understandable, please do ask if you are struggling with any one step.
1. go into your files app on mobile
2. find the folder of your save file, this can be done by clicking the buttons on screen as labelled: 'on my iphone' -> 'stardew valley' -> and finding the folder that has the same farm name as your save file in game (do not enter this folder though)
3. Select this folder by clicking the three dots within a circle icon on the top right, pressing 'select', and clicking that same folder
4. press the 'upload' button on the bottom left (looks like a square with an arrow coming out from the top), then select any app to transfer the file, I use the mail app, but discord, whatsapp, file sharing apps, or anything else work too, just anything you also have access to on the computer.
5. Send this file to yourself (so you can access it on your computer, you could also send it to someone you know and access it from your 'sent' folder or equivalent on other apps)
6. On your computer now, download the file, then drag it into your save file folder (this will only exist if you have opened stardew valley on your pc before). It can be found by pressing windows key+ R ->typing in %appdata% -> opening the folder labelled 'stardewvalley' -> opening the folder labelled 'saves'
And that's it, while it seems like a fair few steps, it become smuch easier once you do it the first time and will basically just be the steps of sending, then dragging it into your save folder. This same process works the other way too, so you can play pc files on mobile.