There is a free application on the Google Play store called Software Data Cable:

Once you've installed it, turn on your WiFi connection and launch the app.

Tap on the "start service" button.

It creates a FTP server on the android device that lets you connect from any computer by opening the resource:

ftp://192.168.1.xxx:8888

where 192.168.1.xxx is the IP address of your device.

You can now copy/paste files over WiFi from/to your android device.

PS - on ubuntu 18.04 get into nautilus and hit + Other Locations then at bottom enter ftp URL above into Connect to Server

My favourite application is SSHelper. It is free on google play store and well documented.

It creates a SSH server you can access using sftp. You mount the filesystem using Nautilus.

http://arachnoid.com/android/SSHelper/

To make it more interesting; You may indeed use AirDroid, which is an android App allows you to connect to your phone interface using a web UI.

Airdroid Supported Instances

1. Connecting via network (you have to be on the same network)


2. Connecting via hotspot (device will create a network which is going to be the same network)


3. Over the internet (you will need an airdoid account)

Usage (using either same network or via device's hotspot)

• Download the AirDroid app from Google play & Open the app


• When it is connected over WiFi or on hotspot, it will display a URL to connect to;

• Connect the given URL from your browser which will ask to allow with a prompt on your device

• once connected your device will displayed with a below message

• and the PC will give you the rest of the information

What can I do with Airdroid

There's a lot you can do with Airdroid (even accessing your folder directories, not sure you need root or not)

1. Transfer file between phone to PC or vise verca


2. Find phone


3. Read notification


4. Operate phone calls, SMS... & many more
   

Hope that helps and clears your question!

There is an Android application named AirDroid . You can download it from the play store. By using it you can view your android's files in ubuntu using a web browser. Here is the link : Airdroid.

Another alternative would be using KDEConnect. You will also need to install the KDE Connect application from the Playstore (or F-Droid). If you are not using KDE on Ubuntu, you should also install the KDE Connect indicator.

You will also get the benefit of receiving your phone notifications on the desktop. You can connect multiple android phones and linux desktops. The connection is encrypted and all involved software is open source.

You can get an idea of the available options in the screenshot below:

I think the simplest solution would be to allow your android to access a directory on your PC.
You could make a samba share on Ubuntu by following e.g. this guide and then use Amaze (Opensource), ES File explorer (Proprietary) or Ghost commander (Opensource) on android.

Another opensource alternative, supporting Linux, Android, Mac OS and Windows is nitroshare:

It is included in the Ubuntu repositories since Ubuntu 16.04:

sudo apt-get install nitroshare

For more up-to-date packages (highly recommended):

sudo apt-add-repository ppa:george-edison55/nitroshare

sudo apt-get update

sudo apt-get install nitroshare

The android app can be found in F-Droid and the playstore.

However, if compatibility with Windows or MacOS is not important, Kdeconnect is the way to go as it is more stable.

In android play store search for an FTP server, the one from The Olive Tree is great ... in app just hit button to start ftp server on your phone which will display a URL like ftp://192.168.1.103:2221 then over on your computer connect to that URL

... on Ubuntu 18.04 in nautilus hit + Other Locations

then enter ftp address in field Connect to Server ftp://192.168.1.103:2221

I use Amaze file manager on Android and FileZilla on Ubuntu. Using Amaze you can set up an FTP server on Android and send and receive files using FileZilla.

1. 
   

   Open Xender in your Android:

   
   
   

   [1

   
   


2. 
   

   Go to Settings < Connect to PC:

   
   
   

   

   
   


3. 
   

   Follow the instructions:

   
   
   

   

   
   


4. 
   

   Add address in Firefox browser:

   
   
   

   

   
   


5. 
   

   Go share your file wirelessly:

   
   
   

   

   
   

GSConnect

If you liked the KDEConnect solution mentioned in another answer, but got scared by the KDE part, there is a JavaScript port of KDE connect without any Qt dependencies.
It has the same features, but as a GNOME Shell extension requires the GNOME desktop to work

On your Android phone, you will have to install the KDE Connect app from F-Droid or the playstore.