Module development and code questions

I want to develop a Payment Gateway for Drupal Commerce 8. But I don't really understand the difference between a Payment Gateway and a Payment Method. What are the differences? What belongs where and is there any payment module available to see a good example?

Edit: I am talking about the classes

Drupal\commerce_payment\Plugin\Commerce\PaymentMethodType\PaymentMethodTypeBase 

and 

Drupal\commerce_payment\Plugin\Commerce\PaymentGateway\OnsitePaymentGatewayBase

I want to redirect a URL like https://example.com/index.html#services to https://example.com/clientsin .htaccess file in Drupal 8. Kindly help me.

I have a php7.1 server running with Drupal7.63. Drupal is not able to connect to any external servers via ssl.

Cannot connect to Solr server:

SearchApiException: "0" Status: : Error opening socket ssl://mydomain.com:18000 in SearchApiSolrConnection->checkResponse() (line 546 of /drupal/sites/all/modules/contrib/search_api_solr/includes/solr_connection.inc)

Cant connect to formassembly server:

Hi Community of Drupal,

I am facing a problem regarding the settings for the e-mailForm after purchase.

It is not in the right order. For now it is:
-product,
-total (and this ammount is price including shipment) 
-shipment.

How can I make the correct order, like first product, then shipment and "total" last?

I built a form table. Each row has a delete button. In the last row of the table there's an add button. Although the buttons are almost identical, the add button works with ajax, the others don't.

I have a few requirements that I can't find an obvious way to complete with contrib modules so am looking at rolling my own.