I tried to install .deb package as mentioned on https://www.mongodb.com/docs/mongodb-shell/install/#std-label-mdb-shell-install
but on running
wget -qO - https://www.mongodb.org/static/pgp/server-6.0.asc | sudo apt-key add -

it says

Warning: apt-key is deprecated. Manage keyring files in trusted.gpg.d instead (see apt-key(8)).

the I did
curl https://www.mongodb.org/static/pgp/server-6.0.asc
cat server-6.0.asc | gpg --dearmor > server-6.0.gpg
sudo mv server-6.0.gpg /usr/share/keyrings/
sudo echo "deb [signed-by=/usr/share/keyrings/server-6.0.gpg] https://repo.mongodb.org/apt/ubuntu focal/mongodb-org/6.0 multiverse" | sudo tee /etc/apt/sources.list.d/mongodb-org-6.0.list
sudo apt-get update
and then finally
sudo apt-get install -y mongodb-mongosh
and everything working fine

db.test.insert({"_id":1, "marks.m1" : 78})
it is insert with no error in mongo shell but when ever i find data with this (.)dot sign field then it gives null
like
db.test.find({"marks.m1":78})
here i am not talk about embedded field. but i talk about just field name.