There was a recent thread about deleting mail in gmail in the alt.comp.software.thunderbird usenet group. I can't find a web archive of it to link to, so I'll quote the relevant posts.

The thread was "question about moving msgs to local folder for Gmail account" started on 2021-09-12 16:40 (UT+1, probably)

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OP:
I have an account with Gmail that I typically access only via
Thunderbird. I am using IMAP. I do understand the Gmail concept of
labels rather than folders.

In Thunderbird, when I look at the account, it shows 32 messages in the
Inbox and 140 messages in Trash.

For the first time in a long time I logged into the account via the
Gmail web interface and saw 32 messages in Inbox and 140 in Trash.
However, there were also over 10,000 messages in All Mail...

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Post by VanguardLH:

All Mail is a history or archive folder. It contains all the e-mail you
have ever sent or ever received. All messages are in the All Mail
folder. Deleting a message from a folder merely has Gmail remove the
label for that folder, but does not delete the message from the All Mail
archive. Deleting from All Mail (if your client subscribes to that
"folder") is useless: the messages will reappear.

If you don't want to keep messages around other than those you overtly
keep, clean the All Mail folder; however, deleting them using a local
e-mail client has them reappear in the All Mail folder. Use their
webmail client. Alas, EVERY message is linked to the All Mail folder,
so when you clean it then you lose all your messages in every folder.
You have to select which messages to move from All Mail to Trash (and
then purge Trash). You can use the Starred feature in their webmail
client. Star those messages you want to keep. Probably easier to go to
other folders to star the messages there instead of hunting for them in
the mess in the All Mail folder. Go to the All Mail folder, and use
"-is:starred" in the search field to find all unstarred messages.
Select all the unstarred messages, and delete them (move to Trash).
Afterward, in the All Mail folder, search on "is:starred" and unstar all
the retained messages (so you can use this trick again later).

Some e-mail client subscribe to the All Mail folder, some don't. If
yours does, you could trash all messages in the All Mail folder, and
then purge the Trash folder to permanently delete the messages, but
you'll lose all messages in every folder (All Mail is an archive for all
messages regardless of what other labels were assigned to the messages).
The Star feature won't be available in your local e-mail client to let
you use that trick to retain messages, because you won't be able to do
the -is:starred search in the All Mail folder. In a local e-mail
client, you have to come up with whatever scheme it can provide for you
to select all messages in the All Mail folder except those you want to
keep (that are shown in other folders, like Inbox, Sent, etc), delete
those messages from All Mail to move them to Trash, and purge the Trash
folder to permanently delete those messages.

All messages in All Mail consume storage quota. All messages in the
Trash folder consume storage quota. When you select a message (using
their webmail client), the action is just called Delete, because that
action moves the message (adds a label) into the Trash folder, so it is
still in your account. When you select messages in the Trash folder,
the option becomes Delete Forever, because the messages get permanently
deleted from your account (they will disappear from All Mail). Because
all/archived messages stay in All Mail, you could end up consuming all
your Gmail storage quota while all your other folders look empty or have
few messages. Only deleting messages from the Trash folder has them
removed from the server.

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The 42-post thead also talks about how gmail handles POP. If you want to read it, well, Thunderbird still handles usernet. Get a free account from Eternal September.