Jegantha, the Wellspring

Once you put your companion into your hand, it behaves like any other card you’ve brought into the game. For example, if it’s countered or destroyed, it’s put into your graveyard, remaining in the game.

Scryfall — June 01, 2020

Wizards of the Coast has issued functional errata for the Companion mechanic. Instead of casting companions from outside the game: Once per game, any time you could cast a sorcery (during your main phase when the stack is empty), you can pay {3} to put your companion from your sideboard into your hand. This is a special action, not an activated ability. It happens immediately and can’t be responded to. It can’t be countered or stopped by cards like Phyrexian Revoker. For more information please see https://magic.wizards.com/en/articles/archive/news/june-1-2020-banned-and-restricted-announcement

Scryfall — June 01, 2020

You can spend mana from Jegantha's mana ability to pay for a hybrid symbol such as {2/W}, but only if you choose to pay the colored mana component, not the generic mana component.

Wotc — April 14, 2023

A generic mana cost is usually represented by numeric mana symbols ({1}, {2}, and so on) and also {X}. It is any cost requiring mana where that cost isn't {C}, {W}, {U}, {B}, {R}, or {G}.

Wotc — April 17, 2020

Jegantha's companion ability compares the exact symbols in the mana costs of cards in your deck. If any one card has the same symbol twice, such as {X}{X}{R} or {(r/g)}{(r/g)}, the companion condition isn't satisfied.

Wotc — April 17, 2020