Drupal 8 url after installation using drush site-install I used Composer to download and install Drupal 8.8.1 on Linux and used drush site:install command for setup. 

Only the following lines/messages were shown after I entered drush site:install command.

You are about to DROP all tables in your 'drupal8' database. Do you want to continue? (yes/no) [yes]:

&gt;yes

[notice]Starting Drupal installation. This takes a while.
[success] Installation complete. User name and password displayed. 


I don't know my site URL. How do I find my site URL? Is there a default site URL?

Thank you
Congratulations on getting this far. You got Drupal and Drush downloaded and were able to configure the database. Those can both be challenging things to do.

Based on your description it seems like do not have a webserver. Drupal.org has some documentation on setting up a webserver and there are a few solutions to that, like setting up apache or nginx. 

Since you mentioned you have drush available one of the easiest solutions is to use drush's runserver command. It is documented here

From the project root run the command:

drush runserver


The output of that command will tell you the URL to load into your browser. By default the URL is http://127.0.0.1:8888 which it will tell you Listening on http://127.0.0.1:8888.

Here's a more detailed article from Lullabot on runserver.