I'm using Termux and youtube-dl python script. Here you have steps to install required software.

1. Install Termux https://play.google.com/store/apps/details?id=com.termux

2. Run Termux and paste following commands:

apt update

apt install python

pip install youtube-dl

Now you can download movies from YT

youtube-dl <link to YouTube movie>

But this is not comfortable way. Here are more steps to make it more user friendly.

1. Create dir in home:

mkdir ~/bin

2. Create file with your favorite editor. My is vim so I used it

vim ~/bin/termux-url-opener

3. Paste following content:

youtube-dl $1

and save file.

4. Make file executable:

chmod 700 ~/bin/termux-url-opener

5. Make Termux internal storage accessible.

termux-setup-storage

Accept access for Termux to internal storage

6. Create config file for youtube-dl:

mkdir -p ~/.config/youtube-dl

vim ~/.config/youtube-dl/config

7. Paste following config:

-o /data/data/com.termux/files/home/storage/movies/%(uploader)s-%(title)s.%(ext)s

Now if you want to save movie from YT simply open YouTube app, find movie, tap Share and choose Termux.

Movie will be saved in Movies directory in internal storage.