Pro tip: You can host FTP servers from your phone. You can access that from your computer's file manager and just drag and drop things on and off of your phone.
I also use Total Commander, latest versions also include WiFi Web download for easy transfer for other devices. Ghost Commander also seems nice, going to use it for a bit.
It's not open source but it's quite literally the best Android application I've ever seen, by a significant margin. It's from the good old days where apps were small, very well designed, very polished and did not collect data.
It's FX, a gem ever more lost to time. The site is a bit old and unmaintained so SSL certificates may give you a bit of trouble, but you can just ignore the warnings, it's just an info page.
FX, despite not receiving updates for years has so many features of such great quality, I've yet to find any alternative with a truly competing interface and feature set.
Been using it for 10 years and it has been the pinnacle of Android.
Edit: I know you'll be detracted from trying it since it's not FOSS, but seriously give it a shot.
I use material files too cause its been suggested a lot in r/degoogle in reddit. But I'm not sure if its still maintained. Anyone has a better app which is also FOSS?
Just no new releases. I haven't found any bugs and it has all the features I need, so I don't miss anything. When an app is finished, you don't need new releases every week.
I don't know which phone do you use, but I'm generally pretty satisfied with the default file managers which are preinstalled on Android devices (in my case, it's Samsung). Is there anything special you require from the file manager app which can't be done with the default one?
X-plore. Its the only one that lets you open multiple folders at once like you would on a desktop computer. It also is a built in FTP manager and will even replace your need to have separate cloud apps like Google Drive and Dropbox.
The Material Files repo shows the last update was three weeks ago, but the latest version on the releases page and f-droid are from 2022. This is confusing...
FYI material files is maintained, there just hasn't been a release since may 2022. The last update was June 9 which you can see on the git repo https://github.com/zhanghai/MaterialFiles
any app from the 'Simple' suite is always worth checking out. (search 'simple files' on f-droid). i personally prefer the stock grapheneos one, but use Simple gallery
I use Material Files and is so far the best you can get, as fa I know. Didn't seen any better Fime manager over the time and some applications just are not regulary updated, bjt still gets updates over the years. Same thing for Material Files.