How can I use different database backend for certain tables? What do I need to do in order to serve config entities from Mongo and cache from Redis database backends?

So basically:
a) how do I tell Drupal that I want to use different db backend for certain tables?
b) what is needed in regard to drivers for those database backends? Are they available?

This is similar to How can I save/load my Entity from a different database in Drupal 8? but in this case I am interested in entirely different db backend + I want to include also non-entity tables so that annotation approach is not working for me.
That's actually more than one question because the answer is different.

To use redis as the cache, you can use our port on github: https://github.com/md-systems/redis

The README isn't been fully updated yet but the quickstart part tells you how to configure it (after enabling the module):

  $settings['redis.connection']['interface'] = 'PhpRedis'; // Can be "Predis".
  $settings['redis.connection']['host']      = '1.2.3.4';  // Your Redis instance hostname.
  $settings['cache']['default'] = 'cache.backend.redis';


To use redis for the cache tags checksum service, which most cache backends use, you need to override a service, for example in sites/default/services.yml, add the following:

To use the redis cache tag checksum implementation, put this in
sites/default/services.yml or any other yml file that is loaded after
core.services.yml:

services:
  cache_tags.invalidator.checksum:
    class: Drupal\redis\Cache\RedisCacheTagsChecksum
    arguments: ['@redis.factory']
    tags:
      - { name: cache_tags_invalidator }


In the same way, you can replace any service that is defined in the core or one of the module services.yml files with a different implementation, for example some of the mongodb service implementations provided by https://www.drupal.org/project/mongodb.

In general, you don't think in terms of tables but services and API's, and you either replace them directly or by configuration he factory service (cache/key value/queue/...)