We're initially told about the Void at the end of time by Ravonna Renslayer, in Loki S01E05. She indicates that it's "a place on the timeline" where "every instance of existence collides at the same point and simply stops" and that "the end of time is still being written."
RAVONNA RENSLAYER: When we prune a branched reality, it's impossible to destroy all of its matter. So we move it to a place on the timeline where it won't continue growing. Basically, the branched timeline isn't reset. It's transferred.
SYLVIE: To where?
RAVONNA RENSLAYER: A void at the end of time. Where every instance of existence collides at the same point and simply stops.
SYLVIE: Why?
RAVONNA RENSLAYER: I don't know. The dogma states that the end of time is still being written, that the Time-Keepers are transforming it into utopia.
SYLVIE: That's nice. Super believable.
RAVONNA RENSLAYER: Whatever the real reason... nothing ever comes back from there.
Loki (2021) - S01E05 - "Journey Into Mystery"
From what's stated and shown though, it doesn't appear that Ravonna has ever been to the Void herself to witness any of this first-hand. Rather, she's apparently relaying "dogma" that she's been indoctrinated with, presumably by the Time-Keepers.
Later in the episode, Sylvie speculates that the Void might not be the end of time, and that there could be a place beyond it where the TVA couldn't detect you, since -- if time is still being written there -- it'd be impossible to create a nexus event.
SYLVIE: Wait. What about the end of time?
RAVONNA RENSLAYER: It's just a void.
SYLVIE: What if The Void isn't the end? What if there's something beyond it? Hiding in the shadow of apocalypses obscured me from the TVA because I couldn't create a diverging branch there, right? So if all of this is still being written, whatever happens is just a new timeline. It would be impossible to start a nexus event there. You could be completely undetectable.
RAVONNA RENSLAYER: That's it. That's the only place they could be.
SYLVIE: So how do we get past The Void?
Loki (2021) - S01E05 - "Journey Into Mystery"
Miss Minutes brings up a diagram in this scene, in which the Sacred Timeline is depicted as a thin, white, horizontal line up to a certain point, beyond which it spreads outward in a cone-like shape and takes on a myriad of colours.

Loki (2021) - S01E05 - "Journey Into Mystery"
Getting back to Sylvie's speculation, it appears that she was on the money, since at the end of S01E05 and the start of S01E06, she and Loki manage to get past the Void to the Citadel at the end of time, where He Who Remains awaits them.
Going by what he says, everything Sylvie and Loki had experienced prior to their arrival at the Citadel was all part of the Sacred Timeline he'd personally scripted.
HE WHO REMAINS: Been a long journey for you, hasn't it? Lot of running, lot of pain. And you... you're a flea on the back of a dragon. In for one hell of a ride. But you did manage to hang on. Mmm-hmm. I guess that counts for something.
LOKI: I'm not sure you quite understand the situation. You've lost. We found you.
HE WHO REMAINS: Duh. Of course you did.
( Sylvie attacks, He Who Remains evades. )
HE WHO REMAINS: Whoa! A swing and a miss. So, we're still doing that, hmm? Let's get all this out of the way. Okay. Here we go. You can't kill me because I already know what's going to happen. See?
( He Who Remains produces a paper script, on which the conversation he's currently having with them is written. )
LOKI: It's a parlor trick.
HE WHO REMAINS: Okay. Don't you wonder how I'm able to get out of the way just before you kill me?
SYLVIE: No. It's because of that little TemPad you have there.
HE WHO REMAINS: Right. But how do I already have it loaded up with everything I need to know to keep from being killed by you two? It's easy. I know it all. And I've seen it all. Everything you guys did on Lamentis, I saw. All the stuff the TVA didn't know about, I knew. All the scheming, all the... talking. That little look by the lake. Quite sentimental, very touching stuff, by the way.
SYLVIE: No. No, we broke out of your little game. That's how we got here.
HE WHO REMAINS: No. Wrong. Every step you took to get here -- Lamentis, The Void -- I paved the road. You... you just walked down it.
Loki (2021) - S01E06 - "For All Time. Always."
As noted in the block quote above, He Who Remains produced a literal paper script to corroborate what he told them.

Loki (2021) - S01E06 - "For All Time. Always."
He also indicated that time was still ongoing at the Citadel -- counting the passing seconds at one point -- and referred to a "threshold" beyond which the Sacred Timeline hadn't yet been scripted; a threshold that was apparently reached during his conversation with Sylvie and Loki.
HE WHO REMAINS: We just crossed... the threshold. Oh. So, I fibbed. I fibbed earlier when I said I know how everything's going to go. I... I know... I knew... everything up to a certain point, and that point was about... seven, eight, nine, ten seconds ago. But now I have no idea. No idea how the rest of this is going to go.
Loki (2021) - S01E06 - "For All Time. Always."
At that point, the Sacred Timeline -- depicted visually outside of the building they were in -- began to diverge into an ever-growing number of branching timelines, and as a consequence, He Who Remains could no longer predict Sylvie's attacks, which allowed her to kill him.
So, if we take everything shown and stated above, and try to weave it all together into a cohesive whole, the takeaway seems to be that the Citadel at the time of time doesn't literally exist at the point at which time comes to an end, nor is it limited to a single point in time at all. Rather, it exists within an unquantified window of time that begins at some point on the Sacred Timeline when it's all flowing in the same direction, but crosses beyond the point at which it begins to separate out into an ever-growing number of branching timelines.
It's unclear how close the Citadel is to the point at which time genuinely comes to an end, assuming there is such a point in the MCU. It may be that the "end of time" part of the Citadel's full name never truly referred to the end of time itself, but merely the end of He Who Remains' scripted Sacred Timeline, beyond which time evidently continues to flow.
It also follows from all this that the Void at the end of time doesn't exist at the literal end of time itself either, and isn't limited to a single point in time. Rather, it exists within a window of time on the Sacred Timeline chronologically preceding the one within which the Citadel exists.