Isekai manga where MC(s) can freely go back and forth between worlds.

[SeveralSolution]

You guys know any others?
Some examples I know of are
https://mangadex.org/title/34638/sen-no-sukiru-o-motsu-otoko
https://mangadex.org/title/22843/saving-80-000-gold-coins-in-the-different-world-for-my-old-age
https://mangadex.org/title/29407/urasekai-picnic
https://mangadex.org/title/32912/welcome-to-japan-elf-san
https://mangadex.org/title/21854/boku-no-heya-ga-dungeon-no-kyuukeijo-ni-natteshimatta-ken
https://mangadex.org/title/5353/gate-jieitai-kanochi-nite-kaku-tatakaeri
https://mangadex.org/title/47023/takarakuji-de-40-oku-atatta-ndakedo-isekai-ni-ijuu-suru-mari-no-isuteria-shougyou-kaihatsu-ki

I guess these count since they did it once technically
https://mangadex.org/title/44494/isekai-de-cheat-skill-wo-te-ni-shita-ore-wa-genjitsu-sekai-wo-mo-musou-suru-level-up-wa-jinsei-wo-kaeta
https://mangadex.org/title/51883/the-fate-of-the-returned-hero

Not that interested in the restaurant type mangas where the Earth humans don't go out and explore the world
e.g. https://mangadex.org/title/18302/isekai-izakaya-nobu

 

[Regal]

https://mangadex.org/title/19089/100-man-no-inochi-no-ue-ni-ore-wa-tatteiru they're kinda being forced to go back and forth but it still counts

 

[BroederTrapist]

This one came to mind

I Won 4 Billion in a Lottery But I Went to Another World

Its spinoff was already linked.