The rights to Spider-Man are also not held by marvel. Tom Hollands version of the Spider-Man may be at home in the MCU since Civil War but Sony is the production and distribution owner, that means Sony is responsible to decide what happens with Spider-Man and Marvel Studios has to accept that.
That's why Venom's solo film is only half MCU represented. So far it is not quite clear if this version is really located in the MCU. Actually they wanted to separate the whole thing, because the Marvel Studios didn't want to allow this interpretation of Venom.
However, it has to be renegotiated that Tom Hollands version as Spider-Man will stay in the MCU and Sony might want Venom to be part of the MCU.
Beside Spider-Man, Sony of course also has the rights to Morbius, next year there will be a film with Jared Leto, and for example they have the rights to Black Cat, they also have the rights to Miles Morales Spider-Man version and of course they are also responsible for the whole animated series and Videogames. That means that all the characters that originated in the Spider-Man comics belong to Sony.
It can be that one character was also very strong in one or more other Marvel comics at home and therefore Marvel Studios is allowed to use them, but on the big known backups, the rights are at Sony and only as long as Sony is up to it, Marvel Studios is allowed to use them.
And it goes on and on and we jump to another studio, Universal. They also have rights, distribution rights. They don't have the production rights but the distribution rights from Hulk. Didn't you ask yourselves why there was still no Hulk solo film in the MCU?
Marvel is not allowed to release such a film and of course they don't want to give Universal any money and that's why Hulk hasn't had a solo film since 2008. The 2008 solo film with Edward Norton half officially belongs to the MCU but afterwards they decided to continue working without Universal.