Question on effects such as the Swarm Ranger's "Writhing Tide" which grants you a flying speed of 10 ft as a bonus action. Is this added to your movement if you've already moved? Could the ranger, for instance, take their full move, then activate the feature and move another 10 feet?

Here's the relevant rule:

Using Different Speeds

If you have more than one speed, such as your walking speed and a flying speed, you can switch back and forth between your speeds during your move. Whenever you switch, subtract the distance you've already moved from the new speed. The result determines how much farther you can move. If the result is 0 or less, you can't use the new speed during the current move.

For example, if you have a speed of 30 and a flying speed of 60 because a wizard cast the fly spell on you, you could fly 20 feet, then walk 10 feet, and then leap into the air to fly 30 feet more.

So in your example, the ranger could not fly after using their full move.

At first read I thought "that can't be right!".... but yeah, Writhing Tide is a Bonus Action that grants you a new movement mode and speed value for that mode, not a Bonus Action that grants you additional movement in the current turn like Dash, or which itself moves you a certain number of feet regardless of your remaining movement for the turn like a Storm Sorcerer's Tempestuous Magic. 

Writhing Tide

7th-level Swarmkeeper feature

You can condense part of your swarm into a focused mass that lifts you up. As a bonus action, you gain a flying speed of 10 feet and can hover. This effect lasts for 1 minute or until you are incapacitated.

Dash

When you take the Dash action, you gain extra movement for the current turn. The increase equals your speed, after applying any modifiers. With a speed of 30 feet, for example, you can move up to 60 feet on your turn if you dash.

Any increase or decrease to your speed changes this additional movement by the same amount. If your speed of 30 feet is reduced to 15 feet, for instance, you can move up to 30 feet this turn if you dash.

Tempestuous Magic

Starting at 1st level, you can use a bonus action on your turn to cause whirling gusts of elemental air to briefly surround you, immediately before or after you cast a spell of 1st level or higher. Doing so allows you to fly up to 10 feet without provoking opportunity attacks.

I think those three cover the three main types of movement ability: grant you additional movement based on your speed without altering your speed (Dash); grant you additional movement not based on your speed which doesn't cost movement (Tempestuous Magic); or grant you additional speed or speeds in new modalities without altering your current movement (Writhing Tide). Maybe there's some other variants out there too?

The basic movement rules are:

Movement and Position

On your turn, you can move a distance up to your speed. You can use as much or as little of your speed as you like on your turn, following the rules here.

Your movement can include jumping, climbing, and swimming. These different modes of movement can be combined with walking, or they can constitute your entire move. However you're moving, you deduct the distance of each part of your move from your speed until it is used up or until you are done moving.

To paraphrase/belabor that in a way that is more long winded but possibly easier to understand....

• Speed is a value which creates a pool of "movement" points for you each turn.
• If your Speed is updated mid-turn, your movement pool does update so that both the maximum value and remaining points within the pool reflect that change (in both directions): e.g., if you move 15 feet of your 30 movement, and then get hit by an effect that gives you -15 Speed, you have no movement remaining. 
• If you have multiple speed values in different modalities (Walk 30/Swim 30/Fly 30), you still just have one movement pool that is set by the highest value (30). You can move in any way that you have a Speed value for (so Walk, Swim, or Fly), up to the value of that speed. This character could Walk 30, Swim 30, or Fly 30, or Walk 10 and Swim 10 and Fly 10, or any other combination that adds up to 30.
• For unequal values, say Walk 60, Swim 30, Fly 10, same thing: you've got a movement pool of 60. You cold Walk all 60, or you could fly up to 10 and swim up to 30 and then Walk the remaining 20, etc.
• If you use Dash or something similar which grants you "extra movement", it increases the movement points in the pool and also lets you exceed those caps by the same amount. Dash gives you extra movement points equal to your speed value (so from a movement pool of 60 to 120, in the example above), and then lets you basically use each speed value twice (so can fly up to 10+10, or Swim up to 30+30, or Walk up to 60+60). 
• Standing from Prone "costs an amount of movement equal to half your speed," but doesn't change those speed values. So in the example above, standing from Prone drops your movement pool from 60 to 30, but with that remaining 30 you'd still be permitted to fly up to 10, swim up to 30, or walk up to 60 (though you only have enough points to walk 30). 

Does that generally make sense?