Cercube with extra features!

Credits

Block all advertisements & Enable background playback.
Set default player quality on WiFi & Celullar.
Save videos in high resolution, save videos as audio-only, save public playlists (beta).
And many more...

iSponsorBlock: Skips annoying sponsor ads inside videos. iSponsorBlock is based on SponsorBlock engine. Basically, this is the iOS version of the SponsorBlock extension.
YouPiP: enable YouTube's native PiP. More options are in YouTube Settings => General.
YTUHD: unlock VP9 codec and in effect, enables video quality of 2K and 4K. You can configure YTUHD in YouTube's Settings - Video quality preferences.
YouTube Dislike Return: brings back Dislike counts under YouTube videos using ReturnYoutubeDislike's API.

and many more...!

YTClassicVideoQuality: since YouTube v16.xx, you need one more step to change the video quality. YTClassicVideoQuality brings back the old video quality selector, which is a lot better than the new one.
YTNoHoverCards: offer an option to enable/disable the annoying suggested videos show up at the end of the videos.
YTABGoodies: allow you to disable some YouTube A/B testing features. It is a combination of several tweaks, such as:

YouAreThere: disable "Video paused. Continue watching?" popup in the YouTube app when you play a long video.
YouRememberCaption: make YouTube remember your video caption setting (if not already).
YTNoCheckLocalNetwork: block the Local Network permission popup.

NOYTPremium: remove YouTube Premium upsell alerts.
YTSpeed: add 2.25, 2.5, 2.75, and 3x playback speed
YTMiniplayerEnabler: enable Miniplayer for all YouTube videos.
DontEatMyContent: prevent the notch/Dynamic Island from munching on 2:1 video content in YouTube.
YTShortsProgress: always enable progress bar and scrubbing in YouTube Shorts (iPhone only).
YTABConfig: allow user to control over YouTube A/B testing flags.

any many more...

Cercube: Hide Cast button is not working. (Workaround: Hide cast button in CercubePlus settings)
YTUHD: Stuttering on 4K videos.
YouPiP (iOS 14.0 - 14.4.2): due to Apple's fault, you may encounter the speedup-bug as described here. The bug also happens when you try to play multi-sources of sound at the same time. Enable LegacyPiP is a workaround. Keep in mind that LegacyPiP also removes UHD quality and breaks YouTube Autoplay next. Use it at your own risk!
Not a bug:

The app won't be able to receive push notifications if you use a free developer account to sideload it.
It's impossible to fix deep-link (a.k.a Open in the YouTube app). However, you can use this Shortcuts as a workaround (tested on iOS 14). Credit: RandomAccessMemories#5025

CercubePlus (or you can call it Cercube+) requires iOS & iPadOS 14.0 and later. The latest version of CercubePlus can be found in the Release tab.
For AltStore user: Open in AltStore (v17.49.6-5.3.12) - It will take a while to install because AltStore needs to download the IPA first.
Version info: (Nov 06)

Expand!

Tweaks/App	Developer	Version	Open source
YouTube	Google Inc	17.49.6	✖︎
Cercube	Majd Alfhaily	5.3.12	✖︎
Open in YouTube	CokePokes	1.2	✖︎
YTNoShorts	MiRO92	1.0.2	✔︎
iSponsorBlock	Galactic-Dev	1.0-15	✔︎
BigYTMiniPlayer	Galactic-Dev	1.0-1	✔︎
YTNoHoverCards	level3tjg	0.0.3	✔︎
YTMiniplayerEnabler	level3tjg	0.0.2	✔︎
DontEatMyContent	therealFoxster	1.0.3	✔︎
YTUHD	PoomSmart	1.3.4	✔︎
YouPiP	PoomSmart	1.7.15	✔︎
YTABConfig	PoomSmart	1.4.4	✔︎
IAmYouTube	PoomSmart	1.2.0	✔︎
YTReExplore	PoomSmart	1.0.2	✔︎
NoYTPremium	PoomSmart	1.0.4	✔︎
YTABGoodies	PoomSmart	1.0	✔︎
YTNoPaidPromo	PoomSmart	1.0.0	✔︎
YTAutoFullScreen	PoomSmart	1.0.3	✔︎
YTShortsProgress	PoomSmart	1.0.2	✔︎
Return YouTube Dislike	PoomSmart	1.7.3	✔︎